Discrepancies in experimental measurements of adsorbed oxygen coverage on copper metal surfaces in working Cu/ZnO/Al2O3 catalysts are interpreted in terms of two types of adsorbed oxygen. The first, O(a), is identical with that observed in studies of single-crystal copper surfaces. The second, O*(a), not seen in single-crystal studies, is more strongly bonded to the metal surface. It is suggested that the adsorption sites of O*(a) contain Zn as well as Cu, from surface alpha-brass (copper/zinc alloy) formation during catalyst reduction. Earlier experimental results on O(a) coverages on various supported copper catalysts are re-assessed. Only catalysts containing Zn (or Ga) gave abnormally high coverages: with other supports, basic or acidic, O(a) coverages are less than similar to 0.1.