Much research has been directed towards development of software environments that allow easy construction of building simulation models of widely varying structure and purpose. For example, TRNSYS has been in use for a number of years. Recently, several new such environments have been proposed. In spite of a considerable variation in model description formats among environments, the underlying mathematical models of physical processes are very similar. While one of the principal aims has been to allow easy sharing of models between users of the same environment, it has not been clear how portability was to be provided between different environments. Another objective has been ease of component model definition, in order to encourage modifications and additions to model libraries. This paper addresses both of these issues, by proposing a neutral and natural format for component model expression. The proposed format encourages equation based model definition because such models can be converted to efficient algorithmic form if needed, whereas the converse is not always true. Nonetheless, algorithmic component descriptions are also supported in order to allow reuse of existing models. Other key features of the proposed format are typing and declaration of linkage elements between models, which allow development of compatible component families, and enhance model exchange and reuse. The proposal considers underlying system modeling issues, including hierarchical submodel decomposition and methods for formal model expression that allow automatic translation to various simulation environments. Also discussed are the software tools needed for library maintenance and model translation.