The adhesive interphase formed between brass and sulfur crosslinked rubber is known to be a complex layer of metal oxides, sulfides, and rubber. Hostile aging of this system produces changes in the structure, morphology, thickness, and mechanical properties of this layer. In this work it has been demonstrated that the overall thickness of the sulfide film can be used to characterize the degradation of the adhesive layer. The thickness of the sulfide film was measured from depth profiles of the brass-plated steel surface obtained by using Auger electron spectroscopy with ion sputter etching following cryogenic fracture of the bond. Aging under several conditions indicated an inverse relationship between adhesion and interfacial sulfide thickness, with magnitude dependent upon properties of the rubber compound formulation.