The uptake of hydrogen and oxygen by (Cu, Na)–Y zeolites has been followed kinetically in a volumetric system. The temperature region investigated was between 273 and 773 K. The influence of several parameters on the reducibility of Cu2+ has been investigated: the degree of cation exchange, the presence of Cu2+ complexing ligands, the procedure of sample preparation and degassing. The formation of Cu+ ions is determined mainly by the mobility of Cu2+ ions in the zeolite. The preparational and degassing procedure has a drastic influence on the reducibility of the Cu2+ species. The rate of formation of Cu° is determined by the diffusion of the reactant through the pores. Cu+ ions and small Cu° clusters are reoxidized reversibly, the mobility of Cu+ ions determining the rate of the process. The reoxidation of Cu° external to the zeolite consists of a homothetic attack of oxygen on the Cu° particles.