A series of Pt and Pt-Au catalysts supported on TiO2 has been studied using C3H6 hydrogenation as a probe reaction to determine the composition of the active catalytic surface. The catalysts were characterized by H-2 chemisorption and TEM analysis to determine concentrations of surface Pt sites for TOF calculations and metal particle size distributions, respectively. Similar TOF values for C3H8 formation (approximately 30 sec(-1)) were observed for a monometallic Pt/TiO2 and a bimetallic Pt-Au/TiO2 sample prepared by impregnation from individual salt precursors. In contrast, the TOF for C3H8 formation over a Pt2Au4/TiO2 sample prepared from an organometallic Pt2Au4 cluster precursor was decreased to 0.07 sec(-1), suggesting strong structure sensitivity for the hydrogenation reaction over this catalyst. Characterization results indicate that Pt on the surface of the Pt2Au4/TiO2 catalyst is heavily diluted by Au atoms. In combination with the kinetic results, this suggests that the highly diluted surface ensembles of Pt are too small to effectively catalyze C3H6 hydrogenation, although electronic effects induced by the presence of Au adjacent to Pt sites can not be excluded.