Abstract: The invention relates to a method and a device for measuring the amount/activity of the enzyme catalase before or in connection with bleaching of cellulose fibers preferably in a pulp suspension for the purpose of providing a better and more uniform product quality and a correct dosing of the actual bleaching substance during the bleaching. According to the invention a measurement sample is derived from a predetermined volume of the pulp suspension after or during the bleaching. Further, a determined amount of bleaching substance is added to the sample, which is agitated so that the bleaching substance is decomposed and oxygen gas is generated, which oxygen gas pushes out a certain sample volume from the measurement sample, which sample volume is measured after a determined time and is directly or indirectly converted, e.g. via a simple algorithm, to a value representing the actual amount of catalase.