In the study of the all-uranium redox-flow battery with a high efficiency, electrochemical investigations of the negative electrode reaction, i.e. U(IV)/U(III) of uranium β-diketone complexes, is necessary in aprotic solvents. In our recent studies, the uranium(IV) acelylacetonate, known to show the simplest voltammograms due to a quasi-reversible U(IV)/U(III) reaction at −2.6 V versus Fc/Fc + in the solvent with the small donor number, shows more complicated voltammograms in the solvents with the larger donor numbers such as dimethyl sulfoxide (DMSO). For U 4+ ion without acetylacetone in such solvents, several researchers reported an electrodeposition at around −1.6 to −2 V versus Fc/Fc +, whereas its details have not known at all. Therefore in this study, the electrode reactions of the U(IV)/U(III) and the U(III)/U(0) reaction of U(dmso) 8(ClO 4) 4 were investigated by direct monitoring of weight changes of a Au electrode during potential sweeps by using the EQCM, as well as the HMDE. Also, an inhibition of the uranium electrodeposition by an addition of the acetylacetone was investigated.