The role of adsorbed atomic oxygen in methanol synthesis is investigated by a series of transient experiments of the interaction of CO and CO2 with a ternary Cu/ZnO/Al2O3 catalyst under methanol synthesis conditions. In particular, the response of adding CO and CO2 as pulses and as steps to the reaction gas mixture is studied. Hereby it is possible to study both the formation Of CO2 from the reaction of adsorbed atomic oxygen (O-*) with CO, and the dissociation of CO2 in situ, i.e., while the catalyst is producing methanol. The experiments show no evidence of a significant coverage of O-* under methanol synthesis conditions. In addition, it is shown that CO2 is the main carbon source in methanol synthesis under the given conditions.