The first steps of CO activation and hydrogenation reactions on Rh catalysts have been studied in order to identify possible intermediates in the reaction pathway to C1 compounds. Rh catalysts were prepared by supporting Rh4(CO)12 On ZrO2 or on ZrO2 after adsorption of Mo(CO)6. The Rh content on all catalysts was 1% (w/w) while the Mo/Rh atomic ratio ranged from 0.0 to 2.0. The supported catalysts were decomposed in H-2 at 523 K (LTR) and at 773 K (HTR). The samples were characterized by TEM, EPR, FTIR, and chemisorption of H-2, O2, and CO. The catalytic activity was tested in the CO hydrogenation reaction at 493 K and 101 kPa. After the thermal treatment Mo was present on the surface as Mo(V) and Mo(VI). The total CO conversion increased with Mo/Rh ratio on LTR catalysts while it was unaffected on HTR catalysts. By increasing the Mo content the amount of CO2 formed increased significantly. The CO carbon efficiency to methane and total hydrocarbon without CO2, did not depend on the Mo/Rh ratio for LTR catalysts while a constant decrease was observed for HTR catalysts. The carbon efficiency to methanol increased with the Mo content both on LTR and HTR catalysts, and that to ethanol changed with the Mo/Rh ratio and the activation temperature. Mo and Rh form a very complex interacting system : Mo inhibits the sintering of highly dispersed Rh obtained by Rh4(CO)12 decomposition, and in the presence of hydrogen Rh promotes the reduction of MoO3 with the formation of Mo bronzes. The environment of Rh particles is strongly modified by the presence of Mo as evidenced by modification of the IR spectrum of chemisorbed CO both in the linear and in the bridged Rh-CO region. On the basis of the experimental results the reaction paths to CO2, CH3OH, and CH4, and the effect of MoO3 promoter are discussed in detail. The methane formation through different pathways involving a bicarbonate and/or a formate intermediate are presented.