In modern enterprises, almost all data is stored in relational databases. Additionally, most enterprises increasingly collaborate with other enterprises in long-running read-write workflows, primarily through XML-based data exchange technologies such as web services. However, bidirectional XML data exchange is cumbersome and must often be hand-coded, at considerable expense. This paper remedies the situation by proposing RELAXML, an automatic and effective approach to bidirectional XML-based exchange of relational data. RELAXML supports re-use through multiple inheritance, and handles both export of relational data to XML documents and (re-)import of XML documents with a large degree of flexibility in terms of the SQL statements and XML document structures supported. Import and export are formally defined so as to avoid semantic problems, and algorithms to implement both are given. A performance study shows that the approach has a reasonable overhead compared to hand-coded programs.