The modification of chitosan, by grafting of oxo-2-glutaric acid, allows its sorption performance to be increased. This enhancement of uptake ability is observed in overall sorption capacity and specificity in sorbing particular metals. This work focuses on the sorption of uranium (VI) and vanadium (V). The sorption isotherms are studied. The experimental results are described according to the Langmuir and Freundlich models. It was shown that uranium sorption is best described by the Freundlich model, while vanadium sorption is difficult to model. The influence of the particle size, significant in the case of uranium, but not for vanadium, shows that the sorption mechanism is not the same for the two metals : surface control is predominant in the case of uranium, due to the poor porosity of the sorbents. The control of overall sorption capacity is related to the chemistry of the metal and polymer : the appearance of hydrolyzed species and protonation of the polymer.