The NMR data obtained after adsorbing (CO)-C-13 on Pt by partial oxidation of (CH3OH)-C-13 showed a nonlinear variation of the chemical shift and the linewidth with decreasing particle size, more change for average sizes less than 5 nm, which can be explained in terms of the variation of relative populations of different CO adsorption sites. There was also a noticeable increase in the chemical shift of CO adsorbed on carbon-supported Pt catalysts as compared to Pt black samples of similar size, which we attribute to a metal-support interaction in the supported catalysts. (c) 2005 The Electrochemical Society. All rights reserved.