This work deals essentially with the study of the interactions between chitosan and an alkaline-earth metal: 85Sr. We showed that addition of carbonate ions is necessary to observe this interaction, but only for pH values over 11. The use of an appropriate calculation software allowed us to determine the nature of the strontium ionic species present in solution. In order to understand the mechanism of interaction, the influence of various parameters on the complexation was studied. Thus, the interaction was maximum for a CO3(2-) concentration close to 10(-2) mol/l, the lowest ionic strength, the most expanded physical form (lyophilisate) and the lowest degree of acetylation of the polymer. We conclude to the formation of a ternary complex between the ion pair Sr2+ CO3(2-) and the amino groups of chitosan. We also showed the presence of interactions between chitosan and carbonate ions which hinder further interactions with strontium ions.