The present invention relates to a method of oxygen gas delignifying lignocelllulosic pulp material in two stages, where a regulation occurs in that in the first reactor, the pressure in the top of the reactor as well as the level of pulp in the reactor are kept constant with the aid of two regulating valves (1 and 2). The first regulating valve (1) is placed in the top of the reactor and the second regulating valve (2) is placed immediately beneath the level of pulp. The first regulating valve (1) is controlled partly by the gas pressure and partly by the pressure difference measured between a first pressure sensor (3) located in the top of the reactor and a second pressure sensor (4) located in the upper part of the reactor and below the level of pulp. The second regulating valve (2) is controlled by the pressure in the top of the reactor such as to maintain the pressure at a desired level. The major part of the gas is discharged through the first regulating valve (1).
