The electrooxidation of 0.1 M ethanol in 0.5 M HClO4 is studied with differential electrochemical mass spectrometry (DEMS) and cyclic voltammetry (CV) over two different carbon-supported platinum-nanoparticle catalysts (20 wt % metal). As documented by transmission electron microscopy (TEM), the catalysts have similar mean particle sizes but different particle size distributions. One catalyst is composed almost entirely (>99 %) of particles less than or equal to 3.5 nm while a significant fraction (21 %) of the other catalyst is composed of particles larger than 3.5 nm. The presence of the larger particles increases the peak oxidation current density by 116 % (from 0.06 to 0.13 mA cm−2). However, the presence of the larger particles reduces the efficiency for complete ethanol electrooxidation to CO2 from 13 % to 6.4 % over the full potential sweep. These differences in activity and selectivity of the two catalysts are discussed in the context of previously established changes in the bonding strength of reaction intermediates as a function of the catalyst particle size.