Well-ordered ultrathin ZnO(0001) films were grown on Pt(1 1 1) in a layer-by-layer mode. The reactivity of the films as a function of the film thickness and coverage was examined by the CO oxidation reaction at near-atmospheric pressures. At low temperatures (similar to 450 K), CO2 production is found to be much higher on the films of partial coverage than on dense ZnO(0 0 0 1) films and bare Pt(1 1 1). Under reaction conditions, monolayer islands and an entire monolayer film transform into two-monolayers-thick islands, which dominate the surface of the active catalysts. The results provide an adequate structural model for elucidating the reaction mechanism on the oxide/metal boundary at technologically relevant conditions. (C) 2013 Elsevier Inc. All rights reserved.