Deterioration mechanism of epoxy structural adhesives by water absorption was examined. Bonding strength decreased with water absorption time, and hydrolysis of adhesive occurred. Bonding strength correlated with degree of hydrolysis defined using peak intensity ratio in Fourier Transform Infrared Spectroscope (FT-IR). Hydrolysis was confirmed in weld-bonding parts of repossessed car, therefore, deterioration of bonding strength in market was able to evaluate using degree of hydrolysis. It was assumed that the water preferentially diffuses at interface between adherent and adhesive in early stage, and then diffuses from whole edge into adhesive layer. Hydrolysis would proceed according to this water diffusion process, and bonding strength and failure mode would be determined by magnitude relation between bulk strength and bonding strength at interface.