Carbon monoxide chemisorbed via methanol dissociative chemisorption onto commercial fuel-cell-grade carbon-supported nanoscale platinum electrocatalysts has been investigated by in situ subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS). The infrared stretching frequency and the Stark tuning rate (i.e., the slope of stretching frequency vs electrode potential) show a strong dependence on platinum particle size. Five platinum particle sizes were analyzed; with average diameters of 2.0, 2.5, 3.2, 3.9, and 8.8 nm. The infrared stretching frequency was found to increase with increasing particle size, while the Stark tuning rate was found to decrease. These results were correlated with those obtained by using solid-state C-13 NMR (Tong, Y. Y.; ct al. J, Am. Chem. Soc. 2000, 122, 1123-29), showing that the particle-size-dependent variations in the infrared stretching frequency and the Stark tuning rate are due to the variation in the 2 pi* back-donation from metal to CO caused by strong interactions between platinum nanoparticles and the conductive carbon support.