A method was used to measure the liquid-side volumetric coefficient of oxygen mass transfer (k(L)a) in closed, semi-batch pressure reactors used in hydrometallurgical laboratories. In this method, the oxygen pressure was monitored as oxygen was continuously sparged into a pressure vessel containing a sodium sulfite solution. A material balance equation was derived for oxygen in the vessel and the experimental data were fitted to this equation. From the constant parameters of the equation, k(L)a was calculated. The solution in the vessel also contained an appropriate amount of cobalt catalyst so that oxygen was consumed rapidly by oxidation of sulfite to sulfate. Under these conditions, the oxygen concentration in the bulk Liquid phase could be assumed to be equal to zero, Values of k(L)a determined by the method under various conditions were reproduced within 12% deviation from the average values. k(L)a was found to increase moderately with temperature in the range of 25 to 75 degrees C, with an activation energy of 33.09 +/- 1.33 kJ mol(-1). The presence of hydrophobic or hydrophilic solids was found to have a deleterious effect on k(L)a.