The oxidation of sodium dithionite with molecular oxygen in alkaline solutions takes place in two steps : in the first, dithionite oxidizes to sulphite and sulphate, S2O42- + O-2 + 2OH(-) --> SO32- + SO42- + H2O which occurs in the fast reaction regime; within this step the oxygen concentration in the bulk liquid is effectively zero and the kinetics can be followed by the decrease in the solution’s reducing power or by the consumption of base required to keep the pH constant. These results allow us to determine the specific interfacial area. In the second step, sulphite oxidizes to sulphate : No-2 a = - 1/2 d[SO32-]/dr = 1/4 dc/dr which takes place in the hydrodynamic regime without catalytic agents deliberately added and allows us to determine the volumetric oxygen transfer coefficient. However, when this method is applied using air as the gaseous phase, it leads to values of both parameters of around 90% of those that were obtained using pure oxygen. These results may be explained by the wide range of the bubble size distribution. This implies that the actual average driving-force of the oxygen transfer using air is even lower than that obtained with the approximation of perfect mixing used.