The variation of interface structure and composition with heat treatment temperature in Au-GaP contact system are investigated by X-ray photo-emission spectroscopy. The experimental results show that the diffusion and migration between the metal and semiconductor have taken place even if at room temperature. Annealing of Au-GaP contact leads to GaP decomposition in the interface accompanied by rapid atomic interdiffusion. During the increase of temperature the interreaction of Au-Ga atoms is enhanced and AuGa multiphase composition compounds are formed. The interfaces of Au-GaP contact system are an alloying regrowth layer containing metal and semiconductor atoms. The temperature dependence of interface characteristics are discussed from the point of view of Metallurgy.