Abstract: A uranium adsorbent is prepared by reacting a polymer containing nitrile groups with hydroxylamine, wherein the amount of imidedioxime groups formed is relatively larger than that of amidoxime groups formed in terms of the integral values of areas of peaks respectively assigned to imidedioxime groups and amidoxime groups in the .sup.13 C-NMR spectrum chart of the adsorbent. The reaction may be effected: 1) in an organic solvent containing water under substantially neutral or weakly alkaline conditions, or 2) under acidic conditions. The uranium adsorptivity of the adsorbent is improved by hydrolyzing its residual nitrile groups under alkaline conditions.