The present invention relates to a method of and an apparatus for bleaching pulp. The method and apparatus in accordance with the invention are especially suitable for bleaching pulp with ozone at medium consistency, in other words when the consistency is 5 - 25 %, preferably 10 - 15 %. It is characteristic of the invention that ozone is mixed into the pulp to be bleached by means of two fluidizing mixers (14 and 16) and that the foamy suspension of gas and pulp thus obtained is discharged to a reaction vessel (30), in which the suspension is mixed, for example, by a paddle mixer.