Addition of CuO_xto defective ZnO_y(0.92<y<1.00) yielded a dispersed catalyst that produced methanol at 523 K and 2 MPa with a selectivity (relative to CH₄) which was higher than that for a unit area of ZnO_yor CuO_xalone. Copper oxide–zinc oxide polycrystalline films (prepared by ion beam sputter deposition) were as stoichiometric as the dispersed samples with junctions best represented as CuO_x/ZnO_y(withx close to 0.5 and y close to 1.0). Photo-STM reveals for the first time the width and nature of the n/p junction, although the gradient of the depletion zone is shallower on the ZnO side of the junction (which is not expected from recent postulates). Responses to CO and H₂ were more representative of CuO_xthan ZnO_y. The properties of such junctions and their role in CO adsorption, catalysis and sensing is discussed, together with the relationship between catalytic and sensor science. Active sites at such junctions may have been detected as those binding CO₂^– with an IR band at 1557 cm^–1 which decompose to release CO₂ or convert to formyl species (and then methanol) depending upon the prevailing conditions.