The effects of ozone and chlorine dioxide on the structure of hardwood cellulose fibers were studied by chemical methods. Chlorine dioxide had very little effect on the cellulose degree of polymerization (DPv), although 40-50% of the chlorine dioxide charged was consumed. By contrast, ozonation of the cellulosic fibers resulted in a substantial reduction in the cellulose DPv. Increasing the ozone charge increased the extent of cellulose degradation. At an ozone charge of approximately 3 wt % (20 mol equiv/100 g of fiber), a 40% reduction in DPv, as measured by cupriethylenediamine viscosity, was observed. A comparison of the cellulose DPv values obtained for ozonated cellulose fibers reduced with sodium borohydride before the viscosity measurements increased confirmed that the primary reaction of ozone with the cellulose fibers was glycosidic bond cleavage, with only a small amount of cellulose oxidation taking place. A functional group analysis of the ozonated cellulose fibers revealed a slight increase in the amount of carbonyl groups introduced into the fibers. In addition, carbon dioxide was detected, which combined with the lack of change in the carboxyl group content, indicated that the oxidation mechanism likely occurred in a three-step process: formation of the carbonyl groups, followed by oxidation to carboxyl groups, and finally, decarboxylation resulting in glycosidic bond cleavage. (C) 2004 Wiley Periodicals, Inc.