This volume contains the proceedings of two international workshops EPEW and WS-FM held atthe Universit' ede VersaillesSaint-Quentin-en-Yvelines,V- sailles, France, 1-3 September 2005. EPEW (European Performance Engineering Workshop) and WS-FM (Int- national Workshop on Web Services and Formal Methods) were colocated to gather the researchers working across the spectrum of techniques for modelling, speci?cation, analysis and veri?cation of the behavior of computer systems and business processes. This proceedings contains a selection of 20 research contributions, out of 59 submissions, which went through a rigorous review process by international reviewers. We therefore owe special thanks to all members of both program committees of EPEW and WS-FM and their sub-referees for the excellent work they did in the short time they had. Additionally, this proceedings includes four invited papers, by Gianfranco Ciardo (University of California at Riverside), Peter G. Harrison (Imperial C- lege London), Cosimo Laneve (University of Bologna) and Wil van der Aalst (Eindhoven University of Technology). These contributions brought an ad- tional dimension to the technical and the scienti?c merit of these workshops. Finally,ourthanksgototheUniversityofVersaillesSaint-Quentin-en-Yvelines, its Laboratoire PRiSM and the CNRS for hosting the workshops and providing technicaland?nancialsupport.