Benzoylchitosans (BzCses) with a different degree of benzoylation were synthesized as membrane materials having a good durability for the separation of benzene/cyclohexane (Bz/Chx) mixtures. Characteristics of BzCs membranes such as contact angle, crystallinity and degree of swelling were significantly influenced by the degree of benzoylation. The BzCs membranes showed a high benzene-permselectivity for a Bz/Chx mixture of 50 wt% benzene in pervaporation and a difference of the benzene-permselectivity for the BzCs membranes with different degree of benzoylation corresponded to a difference in the physical structure of the membranes based on the characteristics of these membranes. When a Bz/Chx mixture of 50 wt% benzene was permeated through the BzCs membranes, permeation rate increased and benzene-permselectivity slightly decreased with increasing degree of benzoylation. These results are discussed from the viewpoints of chemical and physical structures of the BzCs membranes with different degree of benzoylation.