Web software systems are built using heterogeneous software components. They interact by passing messages that exchange data and activity state information. Such heterogeneous message transfers can be struct red sing the eXtensible Markup Language (XML), which allows a flexible common data exchange. Parsers have been developed to check syntax of component interactions, but there are as yet no techniques for checking the semantic correctness of the interactions. This paper presents a technique for sing mutation analysis to test the semantic correctness of XML-based component interactions. In this paper, the web software interactions are specified using an Interaction Specification Model (ISM) that consists of document type definitions, messaging specifications, and a set of constraints. Test cases are XML messages that are passed between the web software components. Classes of interaction-specific mutation operators are introduced and applied to the ISM to generate mutant interactions and test cases.