The evolution of 4-O-methylglucuronic (GlcA) and hexenuronic (HexA) acids contents during kraft pulping of Eucalyptus globulus was investigated. Different operating conditions were considered in the range of 80-165 degrees C for temperature, 10-45 g of Na2O/100 g of oven-dried wood for effective alkali charge, and 15-75% for sulfidity, with a liquid-to-wood ratio of 8 L/kg. The GlcA content was about 5% in the native wood and decreased continuously throughout the cook. Up to 87% of this component was consumed, being dissolved together with the xylan chain or converted to HexA The latter were detected for temperatures higher than 110 degrees C. The highest HexA. values (similar to 40 mmol/kg of oven-dried pulp) were observed for delignification degrees close to those of industrial bleachable pulps, and the subsequent decrease in their content was only noticeable for lower lignin amounts. The temperature and the effective alkali charge were confirmed to have a strong influence on the HexA content profiles and on the degradation/ dissolution of GlcA, but the effect of sulfidity revealed to be negligible on any of them.