Diffusive permeability coefficients of 15 metal ions (alkali, alkali earth and transition) were measured for water-swollen gel chitosan membranes: for an untreated chitosan membrane and four chitosan membranes crosslinked with glutaraldehyde of concentrations between 0.01 and 1% and coated with a protein, and also for comparison for a commercial Cuprophan membrane. The results were compared with those of nonelectrolytes in terms of the capillary pore and free volume models of solute diffusion through gel membranes. While for Cuprophan the permeability coefficients of the metal ions obeyed the models, those for chitosan membranes were found lower than predicted, the largest deviation being fur the transition metal ions. The order of their permeability coefficients: Cu < Ni < Zn < Mn < Pb < Co < Cd < Ag corresponds to the metal ion-chitosan affinity series, which proves that chelation of these ions by chitosan is responsible for the deviation. Mg, Ca and Ba ions were also found to be retained by chitosan membranes, though to a lesser degree, whereas K, NH4, Na and Li ions exhibited minor deviations from the models, attributed to electrostatic interactions. The diffusive properties of chitosan membranes towards metal ions offer potential fur protection of chitosan-based biological systems against destructive effects of heavy metal ions. (C) 2001 Elsevier Science B.V. All rights reserved.