A process and device for mixing fluid into a pulp suspension of cellulose-containing fiber material, in which the pulp suspension is pumped in through a pump inlet, brought into rotation and, at the desired reaction pressure, mixed with the said fluid while passing through a reaction sector comprising a stator shell, a rotor which is coaxial therewith, and at least one fluid inlet, after which the pulp mixture leaves the reaction sector through a pulp outlet, in which the fluid, via the said fluid inlet, is supplied in the vicinity of the center of rotation of the rotating pulp suspension, where the local pressure in the pulp suspension is lower, due to the centrifugal force increasing radially outwards, then the reaction pressure prevailing at the periphery of the pulp suspension.