The decreases in kappa number of an unbleached alkaline hardwood pulp ofcarpinus betuluswhen performing dilute H2SO4hydrolysis at about 100°C, either after or before oxygen bleaching are the same. The main parameters of this acid hydrolysis are specified by studying the acidification of unbleached alkaline pulp suspensions at room temperature, and the extraction kinetics at temperatures in the range of 88°–110°C by following UV absorbance of the hydrolysis liquors.