Rate measurements, density functional theory (DFT) within the framework of transition state theory, and ensemble-averaging methods are used to probe oxygen selectivities, defined as the reaction probability ratios for O* reactions with CO and CH4, during CH4-O-2 catalysis on Pt and Rh clusters. CO2 and H2O are the predominant products, but small amounts of CO form as chemisorbed oxygen atoms (O*) are depleted from cluster surfaces. Oxygen selectivities, measured using (CO)-C-12-(CH4)-C-13-O-2 reactants, increase with O-2/CO ratio and O* coverage and are much larger than unity at all conditions on Pt clusters. These results suggest that O* reacts much faster with CO than with CH4, causing any CO that forms and desorbs from metal cluster surfaces to react along the reactor bed with other O* to produce CO2 at any residence time required for detectable extents of CH4 conversion. O* selectivities were also calculated by averaging DFT-derived activation barriers for CO and CH4 oxidation reactions over all distinct surface sites on cubo-octahedral Pt clusters (1.8 nm diameter, 201 Pt atoms) at low O* coverages, which are prevalent at low O-2 pressures during catalysis. CO oxidation involves non-activated molecular CO adsorption as the kinetically relevant step on exposed Pt atoms vicinal of chemisorbed O* atoms (on *-O* site pairs). CH4 oxidation occurs via kinetically relevant C-H bond activation on *-* site pairs involving oxidative insertion of a Pt atom into one of the C-H bonds in CH4, forming a three-centered HC3-Pt-H transition state. C-H bond activation barriers reflect the strength of Pt-CH3 and Pt-H interactions at the transition state, which correlates, in turn, with the Pt coordination and with CH3* binding energies. Ensemble-averaged O* selectivities increase linearly with O-2/CO ratios, which define the O* coverages, via a proportionality constant. The proportionality constant is given by the ratio of rate constants for O-2 dissociation and C-H bond activation elementary steps; the values for this constant are much larger than unity and are higher on larger Pt clusters (1.8-33 nm) at all temperatures (573-1273 K) relevant for CH4-O-2 reactions. The barriers for the kinetically relevant C-H bond dissociation step increase, while those for CO oxidation remain unchanged as the Pt coordination number and cluster size increase, and lead, in turn, to higher O* selectivities on larger Pt clusters. Oxygen selectivities were much larger on Rh than Pt, because the limiting reactants for CO oxidation were completely consumed in (CO)-C-12-(CH4)-C-13-O-2 mixtures, consistent with lower CO/CO2 ratios measured by varying the residence time and O-2/CH4 ratio independently in CH4-O-2 reactions. These mechanistic assessments and theoretical treatments for O* selectivity provide rigorous evidence of low intrinsic limits of the maximum CO yields, thus confirming that direct catalytic partial oxidation of CH4 to CO (and H-2) does not occur at the molecular scale on Pt and Rh clusters. CO (and H2) are predominantly formed upon complete O-2 depletion from the sequential reforming steps. (C) 2011 Elsevier Inc. All rights reserved.