Preferential oxidation of CO in H-2 was studied by in situ ultraviolet-visible (UV-Vis) and mass spectrometry on flat model Cu and Cu/CeOx catalysts. The experimental findings were interpreted and compared with the results from density functional theory (DFT) calculations of the adsorption and activation energies for the essential reaction steps on Cu(1 1 1). It was found that oxidation of CO preferentially takes place on Cu-(0) and that no significant H-2 oxidation took place under any of the investigated conditions. The presence of CeOx accelerates Cu-(0)-oxidation which leads to catalyst deactivation. In contrast, CeOx promotes the CO oxidation rate on catalysts that were already oxidized to CuOx. The coexistence of CO and H-2 is important to sustain the stability of metallic Cu and thereby a high rate of CO2 formation. In pure CO/O-2 gas, the metallic phase can only be maintained as long as full O-2 conversion is reached. In pure H-2/O-2, Cu is always partly but never fully oxidized, suggesting that a passivating surface layer is formed. This is also the case for H-2 rich gas mixtures with small amounts of CO and O-2. The most active surface termination, Cu-(0), can therefore not be maintained under the industrially most interesting reaction condition where full conversion of trace amounts of CO in H-2 is required. DFT calculations predict that the dissociative H-2 adsorption is a key limiting step for hydrogen oxidation on the Cu(1 1 1) surface, especially when the low sticking coefficient is taken into account. (C) 2017 The Authors. Published by Elsevier Inc.