AbstractThe behaviour of polymeric solids in contact with different environments is considered. A thermodynamic viewpoint is adopted and worked out in terms of chemical reactions, physical processes and the interaction between the two.It is shown that chemical attack alone does not normally cause serious loss of physical properties, but that a combination of chemical and physical (or physico‐chemical) interactions between polymer and environment can lead to rapid deterioration.An understanding of these processes, and their synergistic effects, is essential in designing against environmental failure.