ONIOM calculations have been carried out to determine geometries, adsorption energies and vibrational frequencies of CO on Au-2 and Au-4-exchanged mordenite catalysts (Au-2/MOR and Au-4/MOR) using DFT with the B3LYP and omega B97X-D functionals. We considered structures with two Al atoms, MOR(TT), with at least one of them at the preferential site T4. To investigate the CO adsorption on Au2/MOR(TT) we examined three different situations: the adsorption of one single CO molecule bridged to two Au atoms (case I), one CO molecule adsorbed on a single Au atom (case II), and two molecules of CO absorbed on two Au atoms, one CO per Au atom (case III). From all the previous theoretically and experimentally proposed species of Au inside the MOR channels, including the ones considered in this work, only the mu(2)-CO-Au-2/MOR configuration, with one CO adsorbed in a bridge form on two Au atoms, exhibits nu(CO) frequencies within the experimental range of 1949-2120 cm(-1) (low frequencies). Increasing the size of the Au particles shifts the nu(CO) to higher frequencies. The values of Delta H-ads decrease when the number of adsorbed CO molecules increases, similarly to what is experimentally observed when CO is adsorbed on Au/TiO2 and Au/Al2O3. Charge analysis shows that the Au-2 in the Au-2/MOR and the Au-4 cluster in the Au-4/MOR are cationic. Based on the present results we conclude that the use of nu(CO) as a probe to determine the Au species in the channels of MOR can only clearly identify Au-2(delta+) species inasmuch as the mu(2)-CO-Au-2/MOR configuration is the only one among several others to exhibit nu(CO) frequencies within the experimental low frequency region. (C) 2016 Elsevier B.V. All rights reserved.