Since the emergence of the Web, the ability to map XML data between different data sources has become crucial. Defining a mapping ishowever not a fully automatic process. The designer needs to figure out whether the mapping is what was intended. Our approach to thisvalidation consists of defining and checking certain desirable properties of mappings. We translate the XML schemas and the mapping intofirst-order logic formalism and apply a reasoning mechanism to check the desirable properties automatically, without assuming anyparticular instantiation of the schemas.