Once tried-and-tested, scientific workflows can be, just like web services, stored in public repositories to be shared and reused by other scientists. In practice, however, the reuse of an existing workflow is frequently hampered by the volatility of its constituent service operations. To deal with this issue, we propose a method for locating service operations that can substitute the unavailable operations in a workflow based on a set of replaceability criteria that we formally define. The proposed method has been implemented as part of a tool for the automatic discovery of substitutes for unavailable service operations, and the evaluation of the proposed method against real world scientific workflows developed under the aegis of e-Science projects demonstrated its practical applicability.