Uranyl sorption performances of chitosan and glutamate glucan, a modified chitosan, have been extensively studied. This work deals with the mechanisms involved in the phenomenon. Infra-red and C-13-NMR studies allow the functional sites involved in the sorption to be identified. Amine sites are the main sorbing sites, but modification of spectra in the range of absorption of carboxylic functions indicates that these functions are also involved in the process. Carboxylic sites act directly by ion exchange or sorption of uranyl ions or by a combined effect with sorption on amine sites.