We focus on reliability analysis for systems designed as workflow based compositions of components. Components are characterized by their failure profiles, which take into account possible multiple failure modes. A compositional calculus is provided to evaluate the failure profile of a composite system, given failure profiles of the components. The calculus is described as a syntax-driven procedure that synthesizes a workflow's failure profile. The method is viewed as a design-time aid that can help software engineers reason about system's reliability in the early stage of development. A simple case study is presented to illustrate the proposed approach.