The electrochemical quartz crystal microbalance technique has been used to obtain coulombic efficiencies for the electro reduction of pure Pt or PtRu self-assembled microspheres and the effect of precursor salt concentration and reduction potential. This technique has been demonstrated successfully to simultaneously obtain in situ data for mass change and current-voltage-charge phenomena. The coulombic efficiency for platinum reduction peaked near unity for a potential of -0.15 V vs calomel and tended to lower efficiency at more positive potentials. Similarly, a Pt precursor salt concentration of 4 mM showed a coulombic efficiency in excess of 90%, while the efficiency decreased as the concentration rose. PtRu mixed systems generally gave similar trends, while the efficiency was consistently lower than that of the pure Pt analogs.