The reduction-reoxidation mechanism for uranium(VI) in acidic perchlorate and nitrate solutions has been investigated by cyclic chronopotentiometry. By analysis of data for the second and third transition times it has been shown that possibly no single mechanism of either the reduction-disproportionation sequence or the e.c.e. mechanism is solely operative. The low order pH dependence, the negative dependence on ionic strength of the formal rate coefficient for the chemical reaction following electron transfer, and the consistently shorter third transition time with respect to the second, suggest interpretation based on the involvement of polynuclear complexes, on overlapping of reoxidation waves, and on the existence of an e.c.e. mechanism only in the reverse (reoxidation) step.