Sorption of U(VI) on sepiolite and HDTMA–sepiolite was studied by investigating sorption isotherms and the influence of pH on sorption percentage P for initial U(VI) concentrations of 0.0001–0.001 mol/dm3. Sorption isotherms were linear both for nitrates and acetate solutions, which was explained as a consequence of the precipitation of U(VI) hydroxycomplexes on the surface of the mineral. pH > 9 was optimal for U(VI) sorption on HDTMA–sepiolite, and this fact was related to the sorption of U(VI) anionic complexes on the positively charged surface of the sorbent. The respective chemical reactions for the sorption of different species were proposed.