When gold is deposited on a clean silicon surface, it reacts with silicon readily even at room temperature. After prolonged time, silicon atoms diffuse out through a gold film and accumulate on the top surface. To study the effects of this phenomenon on the silicide formation kinetics, two kinds of samples were prepared on silicon substrates; one with a gold layer between platinum and silicon layers (Pt/Au/a‐Si/Si substrate) and the other without a gold layer (Pt/a‐Si/Si substrate). After annealing, the samples were investigated with Rutherford backscattering spectrometry. It was found that the rate of Pt2Si formation was the same for both kinds of samples. However, in the case of PtSi formation, the samples with a Au layer showed a faster reaction rate than those without a Au layer. These observations are explained by considering the dominant diffusing species during the formation of Pt2Si and PtSi.