An electrochemical study of the open-circuit deposition of platinum on silicon from fluoride solutions is presented. The main features of the process can be explained using the mixed potential theory, in which the electrode potential is defined by two simultaneous reactions at the electrode surface: a cathodic reaction (platinum reduction and deposition) and an anodic reaction (silicon oxidation and dissolution). The charge is exchanged through the semiconducting substrate. After an energy diagram for the cathodic and anodic sites has been developed, further details of the overall reaction are investigated; a simultaneous process of hydrogen incorporation into the silicon substrate is detected. This parallel reaction is the result of silicon etching at the anodic sites and is possibly enhanced by the presence of the metal on the surface. The electronic properties of the resulting interphase are discussed, together with the implications of these findings for the distribution of Pt nuclei during the deposition process. The results presented indicate that the coupling between the anodic and cathodic half-cell reactions is wider than a simple charge balance through the substrate.