In various aerobic bioreactors including activated sludge aeration tanks, the volumetric mass transfer coefficient K(L)a is frequently used as an estimate of the rate of oxygen dissolution into the liquid phase. The K(L)a measurement in such-bioreactors is widely applied with the aid of sodium sulfite (Na2SO3) as an oxygen-consuming substance used to maintain low dissolved oxygen concentration. In the present study, the effect of the addition of Na2SO3 on K(L)a, determined by an off-gas analysis, was investigated specifically from the viewpoint of variations in the size of air bubbles and the enhancement factor associated with the change in sulfite concentration. Experiments were conducted in a draft-tube bubble column, using a zirconia electrode oxygen analyzer for measurement of the O2 mole fraction in the exhaust gas and a dual electrical resistivity probe for measurement of the bubble size. It was found that the increase in the specific gas-liquid interfacial area, resulting from bubble size reduction effected by Na2SO3 functioning as an electrolyte, is more pronounced than the enhancement of the absorption rate through the interface. The upper limit of Na2SO3 concentration for sustaining physical absorption, in the absence of any catalyst, ranges from 30 to 70 mol/m3, while that for preventing the average bubble size from decreasing is about 15 mol/m3. Furthermore, to secure a reliable K(L)a measurement, the K(L)a value should not exceed 50 h-1 for the liquid depth of 3 m even when the limiting conditions are not exceeded. The off-gas analysis proposed in this study for K(L)a determination is expected to be extremely useful provided that the above conditions are fulfilled, since it only requires moderate addition of the sulfite as the oxygen-consuming substance and will not interrupt the reactor operation as long as oxygen uptake occurs in the system.