The reactions of ruthenium and osmium thin films with their silicon substrates lead to the formation of the following phases: the isomorphous Ru2Si3 and Os2Si3, and OsSi 2. Ru2Si3 forms by a diffusion controlled mechanism from approximately 450 to 525°C; the activation energy is 1.8 eV. Os2Si3 grows by the same process and with the same activation energy as Ru2Si3, but at slightly higher temperatures (for equivalent rates). OsSi2 grows at about 750°C by a nucleation controlled mechanism. With an alloy of ruthenium containing 33 at.% rhodium, one obtains RuSi, which forms by a diffusion controlled mechanism with an activation energy of 2.4 eV from 400 to 475°C, and Ru 2Si3. In the case of the ruthenium alloy, the formation of Ru2Si3 is not diffusion controlled; the controlling process is akin to a nucleation mechanism. Ru2Si3 and Os2Si3 form by silicon motion. This is also true of OsSi2, but the high formation temperature results in some metal motion also during the formation of that phase. The diffusion processes are discussed on the basis of simple ideas about diffusion in intermetallic compounds. The nucleation processes are shown to occur for the phases OsSi 2 and Ru2Si3 (with rhodium), as in other nucleated phases, when the enthalpy contribution to the driving force is close to zero. The observation leads one to anticipate that for most (perhaps all) platinum-like metals the silicides with the highest silicon content should be unstable at low temperatures.