The oxygen reduction activity of thin-film platinum deposits on carbon and its relationship to catalyst microstructure has been investigated using rotating-disk-electrode techniques. The thin-film form of platinum is a viable tool for catalyst study as it can provide intrinsic activity data on finely divided carbon-supported platinum. Sputtered polycrystalline thin-film platinum deposits on carbon (loading from 6 to 200 mug/cm2) have been characterized, and their oxygen reduction activity at 25-degrees-C in 1M sulfuric acid determined. Catalyst characterization consisted of electrochemical-surface-area measurement and establishing grain size, morphology, and lattice parameter using transmission electron microscopy. Thin-film platinum exhibits a reduced lattice parameter at very small grain size. Mass activity and specific activity at 0.9 V vs. reversible hydrogen electrode both decrease with increasing gram size (and decreasing specific surface area). Peak mass activity for oxygen reduction was 25 A/g of Pt, and peak specific activity was 0.070 mA/cm2 of Pt surface. The activity trends are attributed to the changes in electronic and geometric properties.