In this first part of a study dealing with the reduction of metallic oxides by Ar-N-2-H-2 postdischarges, copper oxide (CuO) is studied. The influence of the composition of both the gas mixture and the temperature on the reduction kinetics was measured, respectively, between 473 and 673 K. Measurements of the activation energies of the reduction reactions, together with optical emission spectroscopy in discharge and postdischarge, suggest that the reduction mechanisms of CuO is limited by the diffusion of hydrogen in copper below 573 K and by the dissociation of molecular hydrogen above. Atomic nitrogen can also react, but the reduction kinetics of metallic oxides by this specie is much slower than by H.