The polarographic behaviour of uranium(VI) in aqueous C. 1 N nitric acid medium, in the presence of some polyoxa macrocyclic ligands (15-crown-5, benzo-15-crown-5, 18-crown-6, dicyclohexyl-18-crown-6), at 25°C, was investigated. Both in the absence and in the presence of the polyoxa macrocyclic ligands, the reduction of hexavalent uranium in aqueous medium takes place in two steps, each of which involving the transfer of an electron. Under diffusion-controlled conditions, the two steps of the reduction in acid medium proceed reversibly. The results of the polarographic measurements enabled to estimate the ratio of the stability constants of the complex ions of U(111) and of U(IV) with macrocyclic polyethers.