Three graphite-supported commercial fuel cell grade platinum electrocatalysts with different average particle sizes have been studied by cyclic voltammetry and Pt-195 nuclear magnetic resonance (NMR) spectroscopy. There isa strong correlation of platinum particle size with both NMR and cyclic voltammetric lineshapes. From cyclic voltammetric profiles it is possible to observe the transformation to a single crystallite geometry as the particle size increases from 2.0 to 8.5 nm. The particle-size-dependent NMR spectra give the first conclusive assignment of the signals from platinum atoms at surfaces wetted by electrolyte, and provide an opportunity for comparison with spectra from equivalent catalysts in the gas phase. We have also found that the application of multiple voltammetric cycles to such platinum particles increases the particle size.