Abstract: The present invention relates to a mixer for mixing two fluids, the mixer comprising a power source (5) with a rotating transmission shaft (7), a hollow rotor (6) to be submerged in a first fluid (8) and arranged to rotate with the shaft, wherein at least one end of the rotor comprises an opening or openings for supply of a second fluid (9) to the hollow rotor (6), and jet openings (10) are provided in the side surface of the rotor through which the second fluid (9) is discharged into the first fluid (8) surrounding the rotor. To achieve a better operating efficiency, the mixer has a tight liquid seal (14) shaped to cover and isolate substantially the entire portion of the rotor that is above the first fluid.