diff --git a/inc/lib/uuid.js b/inc/lib/uuid.js
new file mode 100644
index 0000000..08f662f
--- /dev/null
+++ b/inc/lib/uuid.js
@@ -0,0 +1,6 @@
+generate_uuid=function() {
+	return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
+		var r = Math.random()*16|0, v = c == 'x' ? r : (r&0x3|0x8);
+		return v.toString(16);
+	});
+}
diff --git a/inc/myco.js b/inc/myco.js
index e41290f..de74c53 100644
--- a/inc/myco.js
+++ b/inc/myco.js
@@ -162,10 +162,10 @@ show_contributions=function(group,contributor_name) {
   }
   else {
     sum=0;
-    for (idx in contributions) {
-      col_actions='<td><div class="btn-group" data-id="'+contributions[idx].id+'"><button type="button" class="btn btn-default contribution_edit_btn"><span class="glyphicon glyphicon-edit"></span></button><button type="button" class="btn btn-default contribution_delete_btn"><span class="glyphicon glyphicon-trash"></span></button></div></td>';
-      tbody.append('<tr><td>'+contributions[idx].getTitle()+'</td><td>'+contributions[idx].cost+' €</td>'+col_actions+'</tr>');
-      sum+=contributions[idx].cost;
+    for (uuid in contributions) {
+      col_actions='<td><div class="btn-group" data-uuid="'+contributions[uuid].uuid+'"><button type="button" class="btn btn-default contribution_edit_btn"><span class="glyphicon glyphicon-edit"></span></button><button type="button" class="btn btn-default contribution_delete_btn"><span class="glyphicon glyphicon-trash"></span></button></div></td>';
+      tbody.append('<tr><td>'+contributions[uuid].getTitle()+'</td><td>'+contributions[uuid].cost+' €</td>'+col_actions+'</tr>');
+      sum+=contributions[uuid].cost;
     }
     total.html(sum+' €');
   }
@@ -271,7 +271,7 @@ on_show_add_contribution_modal=function(e) {
 
 on_click_add_contribution_btn=function() {
   $('#add_contribution_modal #group_name')[0].value=$('#view-group #group_name')[0].value;
-  $('#add_contribution_modal #edit_id')[0].value='-1';
+  $('#add_contribution_modal #edit_uuid')[0].value='-1';
   $($('#add_contribution_modal h4')[0]).html('Ajouter une contribution');
   $($('#add_contribution_modal #add_contribution_submit')[0]).html('Ajouter');
   $('#add_contribution_modal').modal('show');
@@ -322,12 +322,12 @@ on_valid_add_contribution_modal=function(e) {
     date=new Date();
   }
   
-  if($('#add_contribution_modal #edit_id')[0].value=='-1') {
+  if($('#add_contribution_modal #edit_uuid')[0].value=='-1') {
     group.addContribution(new Contribution(contributor,cost,title,date));
   }
   else {
-    contribution_id=$('#add_contribution_modal #edit_id')[0].value;
-    group.replaceContribution(contribution_id,new Contribution(contributor,cost,title,date));
+    contribution_uuid=$('#add_contribution_modal #edit_uuid')[0].value;
+    group.updateContribution(contribution_uuid,new Contribution(contributor,cost,title,date));
   }
   show_contributions(group,contributor_name);
   $('#add_contribution_modal').modal('hide');
@@ -341,13 +341,13 @@ on_close_add_contribution_modal=function () {
 
 on_contribution_delete_btn_click=function(e) {
   var group=groups[$('#view-group #group_name')[0].value];
-  contribution_id=$($(e.target).parents('div')[0]).data('id');
-  contribution=group.contributions[contribution_id];
-  myconfirm('Etes-vous sûre de vouloir supprimer la participation '+contribution.getTitle()+' de '+contribution.contributor.name+' ?',on_confirm_contribution_delete,null,{'group':group,'contribution_id':contribution_id, 'contribution': contribution} );
+  contribution_uuid=$($(e.target).parents('div')[0]).data('uuid');
+  contribution=group.contributions[contribution_uuid];
+  myconfirm('Etes-vous sûre de vouloir supprimer la participation '+contribution.getTitle()+' de '+contribution.contributor.name+' ?',on_confirm_contribution_delete,null,{'group':group,'contribution_uuid':contribution_uuid, 'contribution': contribution} );
 }
 
 on_confirm_contribution_delete=function(data) {
-  delete data.group.contributions[data.contribution_id];
+  data.group.deleteContribution(data.contribution_uuid);
   groups.save();
   on_contributor_change();
 }
@@ -357,13 +357,13 @@ on_contribution_edit_btn_click=function(e) {
   $('#add_contribution_modal #group_name')[0].value=$('#view-group #group_name')[0].value;
   $($('#add_contribution_modal h4')[0]).html('Editer une contribution');
   $($('#add_contribution_modal #add_contribution_submit')[0]).html('Valider');
-  contribution_id=$($(e.target).parents('div')[0]).data('id');
-  contribution=group.contributions[contribution_id];
+  contribution_uuid=$($(e.target).parents('div')[0]).data('uuid');
+  contribution=group.contributions[contribution_uuid];
   $('#add_contribution_modal #add_contribution_contributor_name')[0].value=contribution.contributor.name;
   $('#add_contribution_modal #add_contribution_title')[0].value=contribution.getTitle();
   $('#add_contribution_modal #add_contribution_cost')[0].value=contribution.cost;
   $('#add_contribution_modal #add_contribution_date')[0].value=moment(contribution.date).format('DD/MM/YYYY');
-  $('#add_contribution_modal #edit_id')[0].value=contribution_id;
+  $('#add_contribution_modal #edit_uuid')[0].value=contribution_uuid;
   $('#add_contribution_modal').modal('show');
 }
 
diff --git a/inc/myco_objects.js b/inc/myco_objects.js
index e490014..e74f839 100644
--- a/inc/myco_objects.js
+++ b/inc/myco_objects.js
@@ -96,7 +96,8 @@ function GroupList() {
 function Group(name,data) {
   this.name=name;
   this.contributors=[];
-  this.contributions=[];
+  this.contributions={};
+  this.deletedContributions={};
 
 
   this.isGroup=function() {
@@ -108,14 +109,15 @@ function Group(name,data) {
     for (idx in this.contributors) {
       contributors.push(this.contributors[idx].export());
     }
-    var contributions=[];
-    for (idx in this.contributions) {
-      contributions.push(this.contributions[idx].export());
+    var contributions={}
+    for (uuid in this.contributions) {
+      contributions[uuid]=this.contributions[uuid].export();
     }
     return {
       'name': encodeURIComponent(this.name),
       'contributors': contributors,
-      'contributions': contributions
+      'contributions': contributions,
+      'deletedContributions': this.deletedContributions
     };
   }
 
@@ -156,25 +158,31 @@ function Group(name,data) {
    * Contributions
    */
   this.contributionsByContributorName=function(name) {
-    var ret=[];
-    for (idx in this.contributions) {
-      if (this.contributions[idx].contributor.name==name) {
-        ret.push(this.contributions[idx]);
+    var ret={};
+    for (uuid in this.contributions) {
+      if (this.contributions[uuid].contributor.name==name) {
+        ret[uuid]=this.contributions[uuid];
       }
     }
     return ret;
   }
 
   this.addContribution=function(c) {
-    c.id=this.contributions.length;
-    this.contributions.push(c);
+    this.contributions[c.uuid]=c;
   }
 
-  this.replaceContribution=function(idx,c) {
-    c.id=idx;
-    this.contributions[idx]=c;
+  this.updateContribution=function(uuid,c) {
+    c.uuid=uuid;
+    this.contributions[uuid]=c;
   }
 
+  this.deleteContribution=function(uuid) {
+    this.contributions[uuid].lastChange=new Date().getTime();
+    this.deletedContributions[uuid]=this.contributions[uuid].export();
+    delete this.contributions[uuid];
+  }
+
+
   /*
    * Balance
    */
@@ -186,8 +194,8 @@ function Group(name,data) {
       var sum=0;
       c=this.contributors[idx].name;
       cl=this.contributionsByContributorName(c);
-      for (idc in cl) {
-        sum+=cl[idc].cost;
+      for (uuid in cl) {
+        sum+=cl[uuid].cost;
       }
       if (min==-1 || min>sum) {
           min=sum;
@@ -229,16 +237,21 @@ function Group(name,data) {
           ));
         }
       }
-      if (jQuery.type(data.contributions) == 'array') {
-        for (idx in data.contributions) {
-          this.contributions.push(new Contribution(
-            this.contributorByName(data.contributions[idx].contributor),
-            data.contributions[idx].cost,
-            decodeURIComponent(data.contributions[idx].title),
-            data.contributions[idx].date,
-            idx,
-            data.contributions[idx].lastChange
-          ));
+      if (jQuery.type(data.contributions) == 'object') {
+        for (uuid in data.contributions) {
+          this.contributions[uuid]=new Contribution(
+            this.contributorByName(data.contributions[uuid].contributor),
+            data.contributions[uuid].cost,
+            decodeURIComponent(data.contributions[uuid].title),
+            data.contributions[uuid].date,
+            uuid,
+            data.contributions[uuid].lastChange
+          );
+        }
+      }
+      if (jQuery.type(data.deletedContributions) == 'object') {
+        for (uuid in data.deletedContributions) {
+          this.deletedContributions[uuid]=data.deletedContributions[uuid];
         }
       }
     }
@@ -260,16 +273,17 @@ function Contributor(name,email,id) {
   }
 }
 
-function Contribution(contributor,cost,title,date,id,lastChange) {
+function Contribution(contributor,cost,title,date,uuid,lastChange) {
   this.contributor=contributor;
   this.cost=cost;
   this.title=title;
   this.date=date;
-  this.id=id;
+  this.uuid=uuid || generate_uuid();
   this.lastChange=lastChange || new Date().getTime();
   this.export=function() {
     return {
       'contributor': encodeURIComponent(this.contributor.name),
+      'uuid': this.uuid,
       'cost': this.cost,
       'title': encodeURIComponent(this.title),
       'date': this.date,
diff --git a/index.html b/index.html
index 371eea2..2b9e1d2 100644
--- a/index.html
+++ b/index.html
@@ -270,7 +270,7 @@ body{
       <div class="modal-body">
         <form class="form-horizontal" role="form">
           <input type='hidden' id='group_name' />
-          <input type='hidden' id='edit_id' value='-1' />
+          <input type='hidden' id='edit_uuid' value='-1' />
           <div class="form-group">
             <div class="input-group">
               <span class="input-group-addon">Participant *</span>
@@ -421,6 +421,7 @@ body{
   <!-- Latest compiled and minified JavaScript -->
   <script src="inc/lib/bootstrap/js/bootstrap.min.js"></script>
   <script src="inc/lib/moment.min.js"></script>
+  <script src="inc/lib/uuid.js"></script>
   <script src="inc/myco_objects.js"></script>
   <script src="inc/myco_confirm.js"></script>
   <script src="inc/myco.js"></script>