This book constitutes the thoroughly refereed post-workshop proceedings of the 6th International Workshop on Web Services and Formal Methods, WS-FM 2009, held in Bologna, Italy, in September 2009. The 10 revised full papers presented together with one invited paper were carefully reviewed and selected from 18 submissions. The papers feature topics such as approaches to analyzing and designing systems based on Web Service technology, formal approaches to enterprise systems modeling in general, and business process modeling in particular.