In this article the polarographic behaviours of uranium in phosphoric acid have been studied.In phosphoric acid solutions of moderate concentrations, two distinct waves are observed.These are interpreted as to correspond to reactions the reductions to U^Ⅴ, and to U^Ⅳ.With increasing concentrations of phosphoric acid, U^Ⅴ is less stable and decomposes at the dropping electrode.Therefore the first wave increases, but the total wave height remains unchanged.If the dismutation of U^Ⅴ is complete, or nearly so, then only one wave results, because U^Ⅳ forms stable complex with H₃PO₄, and does not show a reduction wave before hydrogen is discharged.