Abstract: Bleaching device for pulp includes a container to which a supply of pulp and bleaching chemicals containing peroxide as well as discharge of bleached pulp are connected. The device comprises an oxygen measuring device a temperature measuring device and/or a carbon monoxide measuring device, which are arranged to measure oxygen concentration, temperature and/or carbon monoxide concentration, respectively, from the contents of the container such as from the gas volume of the same, or from a substance flow coming from the container, such as gas flow. The measuring device is connected to a data processing unit which is coupled to a controller for automatic control of the process. The process is especially a high consistency pulp bleaching of mechanical pulp, in which the consistency of pulp in the reaction area is at least 25%, advantageously at least 30%.