This volume contains the proceedings of the 5th International Workshop on Frontiers of Combining Systems (FroCoS 2005) held September 19-21, 2005 in Vienna, Austria. Previously, FroCoS was organized in Munich (1996), Amst- dam (1998), Nancy (2000) and Santa Margherita Ligure near Genoa (2002). In 2004, FroCoS joined IJCAR 2004, the 2nd International Joint Conference on Automated Reasoning. Like its predecessors, FroCoS 2005 o?ered a common forum for the presentation and discussion of research results and activities on the combination, integration, analysis, modularization and interaction of f- mally de?ned systems, with an emphasis on logic-based ones. These issues are important in many areas of computer science, such as logic, computation, p- gram development and veri?cation, arti?cial intelligence, automated reasoning, constraint solving, declarative programming, and symbolic computation. There were 28 research papers submitted to FroCoS 2005, authored by - searchers from 16 countries. After extensive reviewing and discussion within the Program Committee, 14 papers including two system descriptions were ?nally accepted for presentation at the conference and publication in this volume. The topics covered by the selected papers include: combinations of logics, theories, and decision procedures; constraint solving and programming; combination - sues in rewriting and programmingas well as in logicalframeworks and theorem proving systems. In addition to the regular accepted papers, this volume also contains papers (three full ones and one abstract) corresponding to four invited talks given by Luca de Alfaro (Univ.