In situ measurements of resistance have been used to study the interaction of Pt with a limited amount of Si deposited in adjacent layers on oxidized Si wafers. One observes in sequence the formation of Pt2Si, Pt 12Si5, and Pt3Si, as seen previously by others using time-sequence annealing. The present method has the advantage of being quite rapid and simple. Changing the heating rate provides one with a measure of the activation energies for several reactions, including, for the first time, values of the apparent activation energy for phases whose formation is dominated by nucleation and growth, rather than by diffusion. A phase transformation was observed during the cooling of Pt3Si. The final Pt3Si phase examined by transmission electron microscopy reveals an interesting Widmanstätten structure. The resistivities of the different phases have been measured: 29, 32, and 53±10 μΩ cm, respectively, in order of increasing Pt content.