Di-o-butyrylchitosan was prepared by reacting chitosan with butyric acid ardiydride in the presence of perchloric acid as a catalyst. C-13-NMR and IR spectra of the modified chitosan suggested that both hydroxyl groups, at the C-6 and C-3 positions, in the chitosan molecules were substituted. The maximum degree of substitution was found to be less than 28%. The results of X-ray diffractograms revealed that, in comparison with the unmodified chitosan membrane, the crystallinity of di-o-butyrylchitosan membranes was remarkably decreased. Meanwhile, it was also observed that the swelling indices of modified membranes were increased significantly in direct proportion to the degree of substitution. Thermogravimetric analysis indicated that the modified membranes exhibited a slightly increased thermal stability compared to the unmodified membrane. The ionic conductivity of di-o-butyrylchitosan membranes after hydration was investigated using impedance spectroscopy. Compared to the unmodified chitosan membrane, the hydrated di-o-butyrylchitosan membrane with a relatively high degree of the substitution showed an increased ionic conductivity of more than one order of magnitude. (C) 2004 Wiley Periodicals, Inc.