In recent years, growing ecological concern has prompted for `design for environment'. One way to achieve this is to design products that are easy to disassemble, because this improves the ability to reuse or recycle parts of a product. This paper presents a computational theory for product modeling and reasoning about product disassembly. This theory, implemented in the PROMOD system, is based on an ontology of different connection types between product components. For the task of reasoning about disassembly, the standard topological relation that expresses that two objects are connected or in contact proves to be inadequate. We therefore introduce, within a topological context, a small number of new ontological primitives concerning the rigidness of connections and the constrained degrees of freedom, which in effect are task-oriented abstractions of geometric and physical-chemical properties of products. On this basis, it is demonstrated that one can automatically ...