We study the problem of substitution of components in a composite service, especially in the setting where the substitute is composed in an asynchronous fashion. By asynchronous composition, we mean that the participants in the composition are not required to synchronize on the input/output actions as long as the input to one participant always follows the corresponding output from another. We show that such asynchronous composition can be realized by synchronous composition of the participating components along with an appropriate buffer process. We obtain the condition which, when satisfied by a service Q1′, allows Q1′ to act as a correct substitute for Q1 in a composition of services Q1 and Q2. Our work extends prior results on substitutability where the conditions relied on synchronous composition and/or on the structural equivalence between the substitute and the component being replaced.