In order to grasp the dynamic behavior of the surface composition of Cu/ZnO-based catalysts, the surface atomic densities of 63Cu and 68ZnO were determined separately with static LEIS on 63Cu/68ZnO/SiO2 catalysts. Our data show that the methanol synthesis activity and surface composition of 63Cu/68ZnO/SiO2 depend strongly on the reduction temperature between 473 and 673 K. The catalyst surface is strongly enriched in ZnO under methanol synthesis conditions. The oxidation state of the Cu species in the outermost atomic layer of the Cu/ZnO/SiO2 surface has been determined by performing LEIS in combination with adsorptive decomposition of N2O. The observed oxidation behavior of the Cu species differs clearly from pure metallic Cu. This oxidation behavior and the methanol synthesis activity of the reduced catalyst surface are explained in terms of the formation of Cu(I)/ZnO with oxygen vacancies and are shown to be clearly affected by the reducing agent (being 5% CO/5% CO2/90% H2 or pure H2).