This book constitutes the refereed proceedings of the 5th International Workshop on Frontiers of Combining Systems, FroCoS 2005, held in Vienna, Austria, in September 2005. The 19 revised full papers presented including 2 system descriptions were carefully reviewed and selected from 28 submissions. The papers are organized in topical sections on combinations of logics, theories, and decision procedures; constraint solving and programming; combination issues in rewriting and programming as well as in logical frameworks and theorem proving systems.