Abstract. We define a formal syntax and semantics for the Rule Definition Language (RDL) of DemaqLite, which is a fragment of the declarative XML message processing system Demaq. Based on this definition, we prove that the termination problem for any practically useful sublanguage of DemaqLiteRDL is undecidable, as any such language can emulate a Single Register Machine—a Turing-complete model of computation proposed by Shepherdson and Sturgis.