The rates and mechanisms of the electron self-exchange between Np(V) and Np(VI) in solution have been studied with quantum chemical methods and compared with previous results for the U(V)-U(VI) pair. Both outer-sphere and inner-sphere mechanisms have been investigated, the former for the aqua ions, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligand. Solvent effects were calculated using the Marcus equation for the outer-sphere reactions and using a nonequilibrium PCM method for the inner-sphere reactions. The nonequilibrium PCM appeared to overestimate the solvent effect for the outer-sphere reactions. The calculated rate constant for the self-exchange reaction NpO2+(aq)+NpO22+(aq) &REVARR; NPO22+(aq)+NPO2+(aq), at 25° C is k = 67 M-1 s(-1), in fair agreement with the observed rates 0.0063-15 M-1 s-1. The differences between the Np(V)-Np(VI) and the U(V)-U(VI) pairs are minor.