the kappa number and viscosity were defined of the incubated pulps the decrease in the kappa number was in a linear way dependent on the decrease in the hexenuronic acid concentration the maximal reduction of hexenuronic acid concentration was 50 meq kg corresponding to a 6 3 unit reduction of the kappa number 90 of the hexenuronic acid being removed the yield of the treatment was 98 calculated on the basis of toc the degradation of hexenuronic acid groups was in accordance with the first order reaction kinetics the minimum retention time reduction of 60 in the hexenuronic acid concentration required by the treatment and the optimal retention time reduction of 80 95 in the hexenuronic acid concentration are illustrated by means of curves fitted to experimental points fig2 at ph 3 0 3 6 the degradation velocity of hexenuronic acid was very close to its maximum value at higher ph values the retention times required are longer due to a slower reaction velocity birch sulphate pulp kappa number 10 3 bleached with oxygen was treated under conditions according to example 8 to remove hexenuronic acid groups the kappa number after the treatment was 5 4 both acid treated and non treated pulp was bleached with ded sequence using several doses of chlorine dioxide and alkali being bleached to the brightness level 88 0 iso the acid treated pulp consumed 2 6 chlorine dioxide calculated as active chlorine and 1 4 sodium hydroxide the corresponding consumption percentages chlorine dioxide and sodium hydroxide by the non treated pulp were 4 3 and 0 8 respectively the yield of the ded sequence was 97 1 for the acid treated pulp and 95 5 for the non treated pulp thus removal.