diff --git a/inc/Storage.class.php b/inc/Storage.class.php
index b08c3b6..78eab6c 100644
--- a/inc/Storage.class.php
+++ b/inc/Storage.class.php
@@ -163,7 +163,7 @@ class Storage {
         }
 
         $query->execute();
-        
+
         $results = $query->fetchAll();
 
         if(count($results) > 0) {
diff --git a/inc/UsersIn.class.php b/inc/UsersIn.class.php
index 10cb347..e449f46 100644
--- a/inc/UsersIn.class.php
+++ b/inc/UsersIn.class.php
@@ -43,6 +43,12 @@
             $this->users_list = $users_in;
         }
 
+        // Check if a user is in a specified invoice
+        // =========================================
+        public function inUsersIn($id) {
+            return in_array($id, array_keys($this->users_list));
+        }
+
         // Maps htmlspecialchars on the class before display
         // =================================================
         public function secureDisplay() {
diff --git a/index.php b/index.php
index a3db930..49c36a1 100644
--- a/index.php
+++ b/index.php
@@ -575,9 +575,43 @@
                         if($user1->getId() == $user2->getId()) {
                             $balances[$user1->getId()][$user2->getId()] = 'X';
                         }
+                        if(!empty($balances[$user2->getId()][$user1->getId()])) {
+                            // If the opposed element in the matrix exists
+                            if(is_float($balances[$user2->getId()][$user1->getId()])) {
+                                if($balances[$user2->getId()][$user1->getId()] >= 0) {
+                                    $balances[$user1->getId()][$user2->getId()] = '-';
+                                }
+                                else {
+                                    $balances[$user1->getId()][$user2->getId()] = -$balances[$user2->getId()][$user1->getId()];
+                                    $balances[$user2->getId()][$user1->getId()] = '-';
+                                }
+                            }
+                        }
                         else {
-                            // TODO : Balances between users
-                            $balances[$user1->getId()][$user2->getId()] = '';
+                            // TODO : Optimize ?
+                            $balances[$user1->getId()][$user2->getId()] = 0;
+
+                            // First, get a list of all invoices paid by user2 and check if user1 was in
+                            $invoices_list_balances = new Invoice();
+                            $invoices_list_balances = $invoices_list_balances->load(array('buyer'=>$user2->getId()));
+                            if($invoices_list_balances !== false) {
+                                foreach($invoices_list_balances as $invoice) {
+                                    if($invoice->getUsersIn()->inUsersIn($user1->getId())) {
+                                        $balances[$user1->getId()][$user2->getId()] += $invoice->getAmountPerPerson();
+                                    }
+                                }
+                            }
+
+                            // Then search for all invoices paid by 1 and check if user2 was in 
+                            $invoices_list_balances = new Invoice();
+                            $invoices_list_balances = $invoices_list_balances->load(array('buyer'=>$user1->getId()));
+                            if($invoices_list_balances !== false) {
+                                foreach($invoices_list_balances as $invoice) {
+                                    if($invoice->getUsersIn()->inUsersIn($user2->getId())) {
+                                        $balances[$user1->getId()][$user2->getId()] -= $invoice->getAmountPerPerson();
+                                    }
+                                }
+                            }
                         }
                     }
                 }
diff --git a/tpl/default_en/css/style.css b/tpl/default_en/css/style.css
index c8e6db5..06ebd14 100644
--- a/tpl/default_en/css/style.css
+++ b/tpl/default_en/css/style.css
@@ -70,6 +70,10 @@ input[type="checkbox"] {
     width: 200px;
 }
 
+.cell-disabled {
+    background-color: rgb(221, 221, 221);
+}
+
 input[type=submit] {
     background-color: green;
     color: white;
diff --git a/tpl/default_en/index.html b/tpl/default_en/index.html
index 5d1c96b..28ac5bc 100755
--- a/tpl/default_en/index.html
+++ b/tpl/default_en/index.html
@@ -18,7 +18,12 @@
             <tr>
                 <th>{$value->getDisplayName()}</th>
                 {loop="users"}
-                    <td>{$balances[$value1->getId()][$value2->getId()]}</td>
+                    {if condition="$balances[$value1->getId()][$value2->getId()] == 'X'"}
+                        <td class="cell-disabled"></td>
+                    {else}
+
+                        <td>{$balances[$value1->getId()][$value2->getId()]}</td>
+                    {/if}
                 {/loop}
             </tr>
         {/loop}