Web services composition is often hampered by various types of data misinterpretation problems. In this paper, we present a comprehensive classification of the data misinterpretation problems. To address them, we develop an approach to automatic detection and reconciliation of data interpretation conflicts in Web services composition. The approach uses a lightweight ontology augmented with modifiers, contexts, and atomic conversions between the contexts, implemented using XPath functions and external services. The WSDL descriptions of Web services are annotated to establish correspondences to the ontology and contexts. Given the naive Business Process Execution Language (BPEL) specification of the desired Web services composition with possible data interpretation conflicts, the reconciliation approach can automatically detect the conflicts and produce the corresponding mediated BPEL by incorporating appropriate conversions into the composition. Finally, we develop a prototype to validate and evaluate the reconciliation approach.