This paper reports on the first phase of the development of the STEP-based Thermal Analysis for Space Application Protocol (STEP-TAS AP), an initiative by the European Space Agency (ESA) in collaboration with the French Space Agency (CNES). The activity responds to the industrial need to have a standard, quality means for the electronic exchange of thermal models between the various companies and organisations involved in space projects and also to improve the possibility to exchange data with tools of other engineering disciplines (notably CAD tools). The development is based on the use of STEP, which is the casual name for the ISO 10303 Standard for Exchange of Product Model Data. The following topics are highlighted: The production of a common description of the information exchange requirements in the space thermal analysis domain, based on work previously initiated by ESA and CNES. Following the STEP methodology this led to the definition of an Application Activity Model and an Application Reference Model. The interpretation of a subset of the thus formulated requirements in terms of the STEP building blocks (Integrated Resources) producing a first version of the Application Interpreted Model. The development of a STEP-TAS software library and a demonstration converter tool between ESARAD and the STEP-TAS physical file (in STEP's neutral text file format). The main conclusion is that the STEP-TAS protocol will enable in the midterm future very comprehensive, industrially robust and tool-independent exchange and archiving of thermal analysis models. On top of that it will also provide a feasible method for connecting thermal CAE tools to the CAx tools of other disciplines. This typically encompasses: import of CAD geometry, solid models, product structure, sharing common parts with mechanical finite element models and models for other space environmental analysis tools (AtOx, Micro-meteorites & Debris, Contamination, etc.).The development of a STEP Application Protocol turned out to be a complex undertaking, but this is justified by the rewards it promises through participation in an open, world-wide industrial standard: the possibility for re-use of well defined building blocks, the availability of robust development toolkits to support the implementation of STEP based software and the growing support for STEP-based product data exchange options by the major CAx package vendors.