This paper focuses on the development of a principled methodology for the dependability analysis of composite Web services. The first step of the methodology involves a UML representation for the architecture specification of composite Web services. The proposed representation is built upon BPEL and introduces necessary extensions to support the second step of the methodology, which comprises the specification of properties, characterizing the failure behavior of the elements that constitute the composite Web services. The automated mapping of this extended UML model to Block Diagrams and Markov models is introduced as the third step of the methodology. A comparative analysis of the aforementioned dependability analysis techniques in terms of precision and complexity is also performed.