A method and an apparatus for mixing of two fluids. The fluid is caused to flow into a mixing chamber (1). The gas is preferably mixed with the fluid before flowing into the mixing chamber. The chamber is vertically oriented and has a mainly cylindrical shape with smooth interior walls. The fluid is led into the chamber via a nozzle (7) in such a way that the fluid flows into the chamber at high velocity. During operation a turbulent gas/liquid phase is formed in the upper part of the chamber and mainly a fluid phase is formed in the lower part of the chamber. The fluid phase with mixed-in gas is drawn off from an outlet (8) arranged at the bottom of the chamber. The mixer could be connected to a transport pipe for fluid.