The electroless deposition of catalysts using hydrazine dihydrochloride or formic acid as the reducing agent in a modified Leaman bath was investigated. The effect of potential on the composition was investigated by potentiostatically depositing thin films on gold from acidic chloride electrolytes at potentials between −0.46 V and 0.34 V (versus normal hydrogen electrode). The physical characteristics and elemental composition of the deposits were determined. An empirical model for the deposition process was developed, taking into account reactant concentration, temperature, and surface potential. The model accurately characterized the deposit composition over a wide Pt/Ru range. The surface potential was estimated to be 0.15 V during electroless deposition using formic acid as the reducing agent based on the empirical model. Deviations from the model were found when hydrazine was used as the reducing agent due to the formation of solution phase ruthenium complexes with hydrazine.