The process of thin polycrystalline Au films dissociation into droplets on the inert Al₂O₃ surface during heating in vacuum was studied. It was shown that this process has no strictly determined temperature. Its melting temperature is essentially lower than that specified in reference books. Nevertheless it was shown that the dissociation process goes through the melting stage and its appearance is conditioned by the thin film surface, and on the whole its temperature reduces when the thin film thickness decreases. However, it was found that as Au film thickness decreases below 20 nm, the process activation energy starts to increase, and the dissociation process temperature also increases. This can be explained by the fact that at lower thicknesses a substrate substantially affects a film behavior.