A new approach for elucidating reaction mechanism of complex reactions, such as Fischer-Tropsch (F-T) synthesis, is presented. It includes a combination of integrated transient and steady-state kinetic modeling, experimental and DFT investigations of kinetic isotopic effects. The integrated transient and steady-state modeling enable the determination of H-2 and CO equilibrium constants and detailed mapping of surface species including surface concentrations and their reactivities. The predictive ability of Langmuir-Hinshelwood type kinetic models has been significantly improved by taking into account the effect of interaction between adsorbed CO on the CO adsorption. Together with DFT investigations of the kinetic isotopic effect, the dominating CO activation pathway through hydrogen-assisted CO dissociation has been confirmed. It led also to a clarification of two carbon pools namely CH2O* (C alpha) and CHx* (C beta) and two corresponding reaction pathways for methane formation. The prevailing reaction pathway for methane formation depends on the operating conditions. Hydrogen surface concentration is the key parameter determining the reactivity of adsorbed CO and the reaction pathways for methane formation. (C) 2013 Elsevier Inc. All rights reserved.