The volumetric mass transfer coefficient was measured in a series of different-sized mechanically stirred reactors, equipped with one to four Rushton six-flat blade turbine impellers mounted on the same shaft, using the sulphite oxidation method. The working volume of liquid in the stirred reactors was in the range of 0.01 m3 to 70 m3. The empirical equations for the estimation of volumetric mass transfer coefficient, were expressed depending on the independent parameters which determine the hydrodynamic conditions of the whole system, namely: impeller speed, gas flow rate and geometrical simplexes in which the whole geometry of the system is included. The advantage of these empirical equations is that for calculating the mass transfer coefficient no information is needed about the gassed power demand.