Service-based distributed computing as a mode of resource sharing is receiving increased attention, motivated both by enterprise applications and by standardization efforts in the grid community. In this context, significant effort is being devoted to service descriptions and related issues, primarily via Web Services and associated technologies. However, it is also critical to enable access to these services via a variety of heterogeneous transport protocols with dynamic runtime flexibility. The RMIX framework is being developed to facilitate the use of heterogeneous transport protocols for access to distributed services via a well-understood and semantically stable language framework, viz. remote method invocation. RMIX permits dynamic negotiation of service-access transport and the selection of the access mechanism that is most appropriate to a given client-service relationship, while retaining a uniform invocation interface. Interoperability issues, the handling of heterogeneous transports, and preliminary experiences with RMIX are described in this paper.