Poisson's ratios of cellulose I-beta and cellulose II were determined for purified ramie and mercerized samples by X-ray diffractometry. Poisson's ratio was 0.38 for [200]/[004] of cellulose I. and 0.30 for [200]/[004] and 0.46 for [110]/[004] of cellulose II. The smaller [200]/[004] Poisson's ratio for cellulose II could be ascribed to closer intermolecular association through hydrogen bonding and van der Waals forces. Poisson's ratio for [1 (1) over bar0]/[004] of cellulose II was estimated to be negative. (C) 2004 Wiley Periodicals, Inc.