Abstract: A method for controlling the combustion and reduction processes in a black liquor (B.L.) boiler of a paper pulp plant by influencing the feeding of black liquor to the B.L. recovery boiler by means of a B.L. gun. Characteristic of the method is that the average thickness of the layer of the B.L. flow, immediately after the black liquor has left the B.L. gun, is controlled to have a desired magnitude. The thickness of the B.L. layer is obtained in the form of an indirect measure which is defined as the droplet index (=DIX) and is constituted by a relationship between the viscosity, the density and the feed rate of the black liquor and by at least the shape and the opening area, of the B.L. gun.