Like vanadium redox couples (V2+/V3+ and VO2+/VO2+), neptunium is a good candidate for the active materials of a redox- flow battery because it possesses two isostructural and reversible redox couples (Np3+/Np4+ and NpO2+/NpO22+). The standard rate constants k(0) of the neptunium electrode reactions were first determined at four types of carbon electrodes. The k0 values for Np3+/Np4+ and NpO2+/NpO22+ are 1.9 x 3 10(-2) and 1.5 x 3 10(-2) cm s(-1), respectively, at the c plane of pyrolytic graphite [PG(c plane)] and also 1.4 x 3 10(-2) and 2.1 x 3 10(-2) cm s(-1), respectively, at plastic formed carbon (PFC). The standard rate constants for the V2+/ V3+ and VO2+/VO2+ electrode reactions were also first determined at PG (c plane) and PFC electrodes. The determined k0 values are larger at the PFC electrode, k(0) = 5.3 x 3 10(-4) and 8.5 x 3 10(-4) cm s(-1) for V2+/V3+ and VO2+/VO2+, respectively, than at the PG (c plane) electrode. There is about a 10(2) order of difference between the k(0) values of neptunium and vanadium ions, and this difference was discussed in reference to the homogeneous electron- exchange rate constants of these redox couples. (c) 2005 The Electrochemical Society. All rights reserved.