Abstract. We discuss how ontologies can be used to facilitate the construction of transformations between XML documents. We introduced a method to compose transformations for XML documents reusing existing (smaller) transformations to define transformations for more complex documents. Linking the types on which these transformations are built to the elements (concepts) of an ontology supports the identification of candidate transformations potentially useful for the composition. If no applicable transformations exist, the ontology can be used for identifying semantically equivalent elements and types of source and target XML-Schema.