TIPR, Towards Interoperable Preservation Repositories, is a project funded by the Institute of Museum and Library Services to create and test the Repository eXchange Package (RXP). The package will make it possible to transfer complex digital objects between dissimilar preservation repositories. For reasons of redundancy, succession planning and software migration, such repositories must be able to exchange copies of archival information packages with each other. Every different repository design, however, describes and structures its archival packages differently. Therefore each type produces dissemination packages that are rarely understandable or usable as submission packages by other repositories. The RXP is an answer to that mismatch. Other solutions for transferring packages between repositories focus either on transfers between repositories of the same type, such as DSpace-to-DSpace transfers, or on processes that translate a specific dissemination format into a specific submission package. Rather than build translators between many dissimilar repository types, the TIPR project has defined a standards-based package of metadata files that can act as an intermediary information package, the RXP, a lingua franca all repositories can read and write. In this paper we present the assumptions and principles underlying the TIPR concept of repository-to-repository exchange, and proceed to describe three aspects of the TIPR project: the RXP format itself; the tests we are conducting to prove and improve the use of the RXP; and finally, issues that have arisen in the course of the project so far.