Unresolved mechanistic details of monomer formation in Fischer-Tropsch synthesis (FTS) and of its oxygen rejection routes are addressed here by combining kinetic and theoretical analyses of elementary steps on representative Fe and Co surfaces saturated with chemisorbed CO. These studies provide experimental and theoretical evidence for hydrogen-assisted CO activation as the predominant kinetically-relevant step on Fe and Co catalysts at conditions typical of FTS practice. H-2 and CO kinetic effects on FTS rates and oxygen rejection selectivity (as H2O or CO2) and density functional theory estimates of activation barriers and binding energies are consistent with H-assisted CO dissociation, but not with the previously accepted kinetic relevance of direct CO dissociation and chemisorbed carbon hydrogenation elementary steps. H-assisted CO dissociation removes O-atoms as H2O, while direct dissociation forms chemisorbed oxygen atoms that desorb as CO2. Direct CO dissociation routes are minor contributors to monomer formation on Fe and may become favored at high temperatures on alkali-promoted catalysts, but not on Co catalysts, which remove oxygen predominantly as H2O because of the preponderance of H-assisted CO dissociation routes. The merging of experiment and theory led to the clarification of persistent mechanistic issues previously unresolved by separate experimental and theoretical inquiries. (C) 2010 Elsevier Inc. All rights reserved.