Abstract. The Message Passing Interface (MPI) is the standard API for high-performance and scientific computing. Communication dead- locks are a frequent problem in MPI programs, and this paper addresses the problem of discovering such deadlocks. We begin by showing that if an MPI program is single-path, the problem of discovering communication deadlocks is NP-complete. We then present a novel propositional encoding scheme which captures the existence of communication dead- locks. The encoding is based on modelling executions with partial orders, and implemented in a tool called MOPPER. The tool executes an MPI program, collects the trace, and builds a SAT formula from the trace using the propositional encoding scheme. Finally, we present experimental results that quantify the benefit of the approach in comparison to a dynamic analyser and demonstrate that it offers a scalable solution.