Self-consistent, periodic density functional theory (DFT) calculations and micro-kinetic modeling are used to compare selectivity for the preferential oxidation of CO (PROX) with respect to H-2 based on studies of elementary reaction steps on the (111) facet of Au, Cu and Pt. The first step of H oxidation (OH formation) has a higher activation barrier than the second step (H2O formation) on all three metal surfaces, indicating that OH formation competes with CO oxidation for the removal of trace amounts of CO from a typical reformate gas. The activation energy barrier for CO oxidation is found to be 0.18 eV on Au(111), 0.82 eV on Cu(111) and 0.96 eV on Pt(111), whereas the barrier for OH formation is 0.90, 1.28 and 0.83 eV respectively. A microkinetic model based on the DFT results shows that trends in the selectivity of these metals at different temperatures is due to (i) differences in the rate constants of the competitive CO and H oxidation reactions, and (ii) differences in the CO and H surface coverages. Our results explain why Au and Cu are more selective PROX catalysts compared to Pt at low temperatures. At higher temperatures, Pt and Cu lose some of their selectivity to CO oxidation, whereas the selectivity on Au decreases substantially primarily because of the significantly weaker CO adsorption.