In order to investigate the electrode reaction of negative electrolyte, U(III)/U(IV), of the all-uranium redox-flow battery proposed by us, we investigated the magnetic susceptibility, spectroscopy and electrode reactions of U(III) and U(IV) complexes with N, N, N′, N′-tetramethylmalonamide (TMMA). The effective magnetic moment for the U(III) complex (1.95 μ B/atomU) is significantly smaller than the theoretical moment for free U 3+ ion, and this may indicate the itinerant nature of U(III) due to the coordination of TMMA. In N, N-dimethylformamide (DMF), the energy levels due to f–f transitions was only slightly perturbed from those of uranium(III) perchlorate in DMF. The U(III)/U(IV) redox potential of U(III)-TMMA complex shifted negatively from that of uranium(III) perchlorate.