CuO is used as a catalyst or catalyst precursor in many chemical reactions that involve hydrogen as a reactant or product. A systematic study of the reaction of H-2 with pure powders and films of CuO was carried out using in situ time-resolved X-ray diffraction (XRD) and surface science techniques. Oxide reduction was observed at atmospheric 2 pressures and elevated temperatures (150-300degreesC), but only after an induction period. High temperature or H-2 pressure and a large concentration of defects in the oxide substrate lead to a decrease in the magnitude of the induction time. Under normal process conditions, in situ time-resolved XRD shows that Cu1+ is not a stable intermediate in the reduction of CuO. Instead of a sequential reduction (CuO double right arrow Cu4O3 double right arrow Cu2O double right arrow Cu), a direct CuO double right arrow Cu transformation occurs. To facilitate the generation of Cu1+ in a catalytic process one can limit the supply of H-2 or mix this molecule with molecules that can act as oxidant agents (O-2, H2O). The behavior of CuO-based catalysts in the synthesis of methanol and methanol steam reforming is discussed in the light of these results.