The early stages of Pt electrocrystallization onto glassy carbon (GC), n-GaAs and n-InP were studied by means of (i) analysis of current-time transients and (ii) surface observations (transmission electron microscopy — TEM). It is shown that the nucleation modes depend on the substrate. As the film growth is only controlled by the overpotential between the metal clusters and the solution, an energy diagram of the interface during deposition is proposed in the case of semiconductors. The origin and the influence of the shifts in the semiconductor band edges are also discussed.