Peroxide bleaching of pulp, for the production of paper and fibrous products, involves measuring the oxygen concentration, temperature and/or carbon monoxide concentration in reaction area, based on which process is monitored

The method involves contacting a cellulose based fibrous pulp (M) with peroxide (C) in a container (T). The oxygen concentration, temperature and/or carbon monoxide concentration in the reaction area is/are measured, and the process is monitored based on the result. The concentrations of oxygen and carbon monoxide are measured by measuring devices (A, V) and temperature is measured by sensor (S), from the gas volume in the upper part of bleaching container or from the vent gas flowing from the bleaching tower. The consistency of pulp in the reaction area is at least 25% preferably at least 30%. An independent claim is also included for the pulp bleaching device for.