A crosslinked N-methylene phosphonic chitosan (=NMPC) was newly synthesized by reacting chitosan with phosphorous acid in order to remove traces of copper(II) from a crude palladium(II) solution. First of all, the adsorption selectivity of NMPC for metal ions was examined from a 1 mol/dm(3) (=M) aqueous ammonium nitrate solution. The adsorption of metal ions on NMPC was dependent on an equilibrium pH, and their adsorption shifted to a lower pH region compared with crosslinked original chitosan (=CLC). NMPC began to adsorb all the metal ions except for palladium(II) at around pH 1-3 and then their adsorption increased with an increase in PH. On the contrary, palladium(II) was adsorbed in lower pH around 0-1, then its adsorption rapidly decreased with increasing pH. These results indicate that base metals can be selectively separated and removed from the crude palladium(II) solution using NMPC only by adjusting the pH. Especially, the adsorption equilibrium of copper(II) was quantitatively examined since the separation of copper(II) from a crude palladium(II) solution is commercially very important. The adsorption of copper(II) showed the maximum value at around pH 5 and then it rapidly decreased in the higher PH region. The adsorption of copper(II) was little dependent on the ammonium nitrate concentration at PH < 4, however, beyond which it decreased with the increasing concentration of ammonium nitrate. The adsorption equilibrium of copper(II) was quantitatively discussed by taking account of the formation of ammine complexes of copper(II) with ammonia in the higher pH region. The adsorption capacity of NMPC for copper(II) was 3.81 mol/kg, which is twofold greater than that of CLC.