When numerous users and applications work concurrently on Extensible Markup Language (XML) documents, the transaction management of these XML documents becomes an important issue. The detection of conflicts between operations becomes vital in the query optimization for a query compiler and concurrency for a transaction manager. XML data require tailor-made conflict detection mechanisms since traditional solutions for the relational model are inadequate. Such mechanisms need to take full advantage of the hierarchical structure of semi-structured data. In this paper, we will solve a fundamental problem of the concurrency control for valid XML documents, which is the detection of conflicts between operations. We will formalize READ-UPDATE and UPDATE- UPDATE conflicts in valid XML documents, and propose effective algorithms to detect such conflict when the update operations are specified using XPath expressions. Furthermore, we will show that our proposed optimistic concurrency control scheme is a solution to the phantom and the pseudo-conflict problems.