In this study, the effect of particle size on the reaction rate and selectivity of preferential oxidation of CO (PROX) reaction was investigated on 2 wt% Pt/-Al2O3 catalysts prepared by an incipient wetness technique. The particle size of the catalysts was modified by calcination temperature and duration. CO oxidation and preferential CO oxidation reaction were studied on these catalysts at lambda = 2 (P-O2/P-CO) = 1. The CO oxidation reaction activation energy decreased with increasing particle size, consistent with previous reports. CO reaction orders exhibited no systematic change with respect to particle size, but a minor but systematic decrease in the CO oxidation reaction order was seen with respect to oxygen partial pressure with decreasing particle size, indicating higher selectivity toward PROX reactions at higher; on larger particles. On catalysts with 69 and 83% dispersions, the light-off curves were identical in hydrogen-free streams, whereas on the catalyst with higher dispersion, the light-off temperature was lower in the presence of hydrogen, indicating that the hydrogen oxidation reaction was more facile on catalysts with more defect sites. (c) 2006 Elsevier Inc. All rights reserved.