The problem of transaction synchronization is important in XMLbased applications based on client-server model, especially when both server and clients are allowed to store data. When clients modify their local database, writing this transaction back to the server and to all other clients requires transaction synchronization. To achieve reliable and efficient transaction synchronization, there is a need for some sort of efficient protocol that can guarantee the synchronization of databases on the server and all clients. In this paper, a protocol is proposed for transaction synchronization using XML database and XML as a transfer medium between the server and clients. Importantly, this protocol can be used in any type of database with XML as a data transfer medium using some kind of database mapping from any database to XML. This protocol significantly reduces the cost of data transfer between server and clients, thus increasing efficiency. Categories and Subject Descriptors H.2.4 [Data...