The objective of this work was to investigate the microstructural dependence of the oxygen reduction activity of carbon-supported thin-film Pt-Ph catalysts in comparison with that of thin-film Pt. Thin-film rotating-disk-electrode (RDE) techniques were applied to arrive at diffusion-corrected, i.e., true activity measurements at overpotentials typical of current generation. Thin-films of Pt and Pt-Rh [80:20 atomic percent (a/o)] were prepared by sputter deposition onto graphite, characterized using transmission electron microscopy (TEM), and tested in 1M H2SO4 at 25 degrees C. The surface of Pt-Rh alloys becomes Pt rich when cycled repeatedly to potentials above ca. 1 V vs. reversible hydrogen electrode (RHE). The lattice parameter of thin-film Pt has been shown to decrease with decreasing grain size while that of thin-film Pt-Ph increases. The latter effect has been attributed to the presence of oxygen in the Pt-Rh lattice. The specific activity at 750 mV(RHE) of thin-film Pt increases with decreasing particle size while that of thin-film Pt-Ph also shows a slight increase and is much lower than Pt. Correspondingly, the mass activity of thin-film Pt-Rh is also less than for thin-film Pt. The lower specific activity of thin-film Pt-Ph is due to its lattice expansion and to the presence, at potentials of interest, of oxide on the surface. The activity of large-grained or heat-treated thin-film Pt-Rh is more like the bulk material which previously has been shown to be less than Pt.