Abstract: A method and apparatus for introducing combustion air in the form of air jets into a furnace (2) of, for example, a soda recovery boiler. Combustion air is introduced through air ports (12), arranged substantially at the same level (10), in air jets (13) of at least two sizes. Air ports in the walls (4) of the furnace are dimensioned in different sizes such that their hydraulic diameters increase from the corners of the furnace towards the centers of the furnace walls, whereby the degree of penetration of the respective air jets flowing through the air ports increases. The penetration of the air jets in maintained constant at different loading conditions.