In the preparation of dissolving sulfate pulp by the conventional batch system, the digestion is about three cycles per day, and the acid treatment (prehydrolysis) and alkaline cooking (sulfate cooking) are both carried out in a single digester. This process could be improved markedly by separating the prehydrolysis process from the alkaline cooking and applying a continuous system for the prehydrolysis.By this way, problems concerning the materials of digester would easily be solved and the digestion .cycle could be shortened considerably.A pilot plant of semi-commercial scale for this continuous prehydrolysis was built and several tests were carried out with satisfying results . Besides the originally desired results mentioned above, it was found that by applying this process time required for the sulfate cooking was also shortened down to one half of the conventional process. The considerable increase of reactivity in the sulfate cooking was attributed to the fact that the internal structure of chips is loosened on the prehydrolysis process in which chips are kept under high pressure and then suddenly blown out into atmospheric pressure.