The effect of sulfur mustard (SM), a well-known chemical warfare agent on the microhardness of two poly(ethylene terephthalate) (PET) films was investigated at different loads. SM induces hardness in PET films, perhaps due to an antiplasticizing effect. Heat treatment of the films enhanced their microhardness. The heat-set films show a further increase in their microhardness after exposure to SM. These results were supported by physicochemical techniques like plasma and amine etching, which revealed complex etching phenomena giving rise to a structure-specific pattern. The film having a higher weight loss due to plasma and amine etching showed lower microhardness.