Abstract: In a method and an apparatus for mixing chemicals into a fibre pulp, the pulp passes in a space between a fixed casing and a rotating device through three treatment sections. In the first treatment section a transport screw is arranged for transporting the pulp and impart to it a pressure up to the subsequent treatment section. In this treatment section both the casing and the rotating device are provided with guide elements which cause shearing between the fibre layers of the pulp by alternating velocity gradiants in the pulp and oscillating movement thereof. In the next treatment section the pulp is imparted a retarding velocity in a first part of the section and thereafter an accelerating velocity in a second part of the section during simultaneous oscillating movement and shearing between the fibre layers. This is accomplished by a configuration of both the fixed casing and the rotating device in the form of double-cones having their basis abutting each other.