diff --git a/CHANGELOG b/CHANGELOG
index 1498617..ede240c 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -5,10 +5,18 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.1] - 2023-01-05
+
+### Changed
+
+- Disabled autocommit when transactions are used
+- Added isolation level serializable to all transactions
+
 ## [1.0] - 2023-01-01
 
 ### Added
 
 - Initial Release of the Project
 
+[1.1]: https://github.com/intragart/Shared-Household-Expenses/compare/v1.0...v1.1
 [1.0]: https://github.com/intragart/Shared-Household-Expenses/releases/tag/v1.0
diff --git a/version b/version
index 472de5a..634650e 100644
--- a/version
+++ b/version
@@ -1 +1 @@
-v1.0
\ No newline at end of file
+v1.1
\ No newline at end of file
diff --git a/webapp/www/api/delete-purchase.php b/webapp/www/api/delete-purchase.php
index 3945daf..ae59ab4 100644
--- a/webapp/www/api/delete-purchase.php
+++ b/webapp/www/api/delete-purchase.php
@@ -27,7 +27,12 @@
     $db_settings = get_db_login("admin");
 
     // connect to database and start a transaction
-    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]); 
+    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]);
+    
+    // disable autocommit and set isolation level to serialize
+    $db->autocommit(false);
+    $db->query("SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE");
+    
     $db->begin_transaction(MYSQLI_TRANS_START_WITH_CONSISTENT_SNAPSHOT);
 
     try {
diff --git a/webapp/www/api/insert-purchase.php b/webapp/www/api/insert-purchase.php
index 0ffd1c6..bd3a47a 100644
--- a/webapp/www/api/insert-purchase.php
+++ b/webapp/www/api/insert-purchase.php
@@ -94,7 +94,12 @@ function getRetailerID ($retailer_name) {
     $db_settings = get_db_login("admin");
 
     // connect to database and start a transaction
-    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]); 
+    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]);
+
+    // disable autocommit and set isolation level to serialize
+    $db->autocommit(false);
+    $db->query("SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE");
+    
     $db->begin_transaction(MYSQLI_TRANS_START_WITH_CONSISTENT_SNAPSHOT);
 
     try {
diff --git a/webapp/www/api/insert-user.php b/webapp/www/api/insert-user.php
index 1084228..413e8a9 100644
--- a/webapp/www/api/insert-user.php
+++ b/webapp/www/api/insert-user.php
@@ -43,7 +43,12 @@
     $db_settings = get_db_login("admin");
 
     // connect to database and start a transaction
-    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]); 
+    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]);
+
+    // disable autocommit and set isolation level to serialize
+    $db->autocommit(false);
+    $db->query("SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE");
+    
     $db->begin_transaction(MYSQLI_TRANS_START_WITH_CONSISTENT_SNAPSHOT);
 
     try {
diff --git a/webapp/www/api/update-purchase.php b/webapp/www/api/update-purchase.php
index 0303ae6..76817b7 100644
--- a/webapp/www/api/update-purchase.php
+++ b/webapp/www/api/update-purchase.php
@@ -110,7 +110,12 @@ function getRetailerID ($retailer_name) {
     $db_settings = get_db_login("admin");
 
     // connect to database and start a transaction
-    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]); 
+    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]);
+    
+    // disable autocommit and set isolation level to serialize
+    $db->autocommit(false);
+    $db->query("SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE");
+    
     $db->begin_transaction(MYSQLI_TRANS_START_WITH_CONSISTENT_SNAPSHOT);
 
     try {
diff --git a/webapp/www/api/update-user.php b/webapp/www/api/update-user.php
index 79a00bc..5ac45ea 100644
--- a/webapp/www/api/update-user.php
+++ b/webapp/www/api/update-user.php
@@ -46,7 +46,12 @@
     $db_settings = get_db_login("admin");
 
     // connect to database and start a transaction
-    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]); 
+    $db = new MySQLi($db_settings[0], $db_settings[1], $db_settings[2], $db_settings[3], $db_settings[4], $db_settings[5]);
+
+    // disable autocommit and set isolation level to serialize
+    $db->autocommit(false);
+    $db->query("SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE");
+    
     $db->begin_transaction(MYSQLI_TRANS_START_WITH_CONSISTENT_SNAPSHOT);
 
     try {