Modeling the evolution of the active phase in terms of shape and composition in supported catalysts as function of the reaction conditions is an important challenge. Herein, we employ methods rooted in genetic algorithms and grand canonical Monte-Carlo simulations to determine relevant structures of a gold clusters placed on a ceria support in oxygen atmosphere. Based on optimized structures, microkinetics simulations identify relevant structures present under the out-of-equilibrium conditions of the ongoing CO oxidation reaction. The results emphasize the role of cationic gold atoms at the interface between gold and ceria, which bind oxygen atoms strongly. At low temperature and low CO/O-2 ratio, the catalytic cycle for a 6-atom gold cluster mainly involves the Au6O3 and Au6O2 couple via the associative O-2 mechanism of CO oxidation. At higher CO/O-2 ratio and elevated temperature, the Au6O and Au-6 couple are the dominant active species. (C) 2020 The Author(s). Published by Elsevier Inc.