The thermal behavior of vinyl ester resin matrix composites reinforced with jute fibers treated for 2, 4, 6, and 8 h with 5% NaOH was studied with Thermo-gravimetric analysis and differential scanning calorimetry. The moisture desorption peak shifted to a higher temperature, from 37 to 58.3degreesC, for all the treated-fiber composites because of improved wetting of the fibers by the resin and stronger bonding at the interface. The degradation temperature of the vinyl ester resin in the composites was lowered to 410.3degreesC from that of the neat resin, 418.8degreesC. The X-ray diffraction studies showed increased crystallinity of the treated fibers, which affected the enthalpy of the a-cellulose and hemicellulose degradation. The hemicellulose degradation temperature remained the same (299.7degreesC) in all the treated-fiber composites, but the enthalpy associated with the hemicellulose degradation showed an increasing trend in the treated composites with a small increase in the weight loss. This could be attributed to the increased hydrogen bonding between the more accessible -OH groups of the hemicellulose in the noncrystalline region of the jute fiber and the resin. The degradation temperature of a-cellulose was lowered from 364.2 to 356.8degreesC in the treated composites. The enthalpy of a-cellulose degradation showed a decreasing trend with a lowering of the weight loss. The crystalline regions of the fiber, consisting of closely packed a-cellulose chains, were bonded with the resin mainly on the surface through hydrogen bonds and became more resistant to thermal degradation; this reduced the weight loss. (C) 2004 Wiley Periodicals, Inc.