A polystyrene-aluminum joint, the adhesion of which was promoted by a silane coupling agent, was examined by fracturing the interface under water at different loads and temperatures. X-ray photoelectron spectroscopy (XPS) analysis of the fractured surfaces showed that the locus of failure was mainly interfacial at low loads, but it gradually moves away from the interface at higher loads. This movement of the failure locus reflects a transition of the mechanism of interfacial debonding from the hydrolysis of the siloxane bonds to the cleavage of the main polymer chains. A rate-dependent bond failure model qualitatively describes the above process, in which the activation energy of bond dissociation is assumed to be a time-dependent parameter.