(57) An apparatus and process for producing chemimechanical wood pulp wherein the mixture is fed from a first tank, into which the product and reagents are continuously fed, to a series of successive tanks of which at least one can be kept substantially empty in turn, the level of the product suspension present in each tank being able to be externally set so that the flow from one tank to another and thus the total product reaction time become controlled in accordance with said level, it also being possible to keep those products simultaneously present in the plant but with different characteristics separate from each other by virtue of said tank which is kept substantially empty in turn.