Quaternary ammonium groups were grafted onto ligno-cellulosic material (cotton, wood sawdust, and maize-cob flour) to obtain a support able to adsorb oxy-anions containing chromium atoms (chromate and dichromate). For both anions in aqueous solutions, adsorption proceeded very quickly and reached a limit value dependent on the rate of ammonium grafting; at saturation, the Cr/N+ ratio was always close to 1/1 for chromate anions, but two ammonium sites were needed to link a dichromate anion in a stable structure, resulting in the same 1/1 Cr/N+ ratio. Tentatives for modeling the adsorption process were performed with the Langmuir, Freundlich, and Jossen equations. A nearly total desorption occurred only when these saturated supports were treated with solutions of high ionic strength (4M potassium chloride).