The feasibility that quaternized chitosan was used as cationic layer for surface modification of two kinds of commercial cation exchange membrane (CEM, one is heterogeneous and the other is homogeneous) to enhance their selectivity was reported in this paper. The membranes before and after modification were characterized by observation of surface morphology and determination of selective separation properties in detail, respectively. Scanning electron microscopy (SEM) displayed the difference of surface morphology induced by modification. The selectivity of membranes toward protons vs. metallic cations was investigated through electrodialysis experiments in binary systems (H+/Zn2+ and H+/Al3+) and tertiary systems (H+/Zn2+/Al3+). The results indicated that the transport of the metallic cations was decreased significantly after surface modification due to the electrostatic and steric barrier created by the thin cationic quaternized chitosan layer. In addition, the contrastive experiments such as the orientation of the modified layer (facing cathode or anode) were also carried out and analyzed tentatively. (C) 2008 Elsevier B.V. All rights reserved.