In this study, the effect of agitation intensity on gas-liquid reactions in variable volume systems is examined by oxidizing sulphite solutions in an aerated stirred tank under semi-batch operation. Batchwise operation results can predict the time courses of the reactant concentration in a constant rate fed-batch, constant rate discharged batch, and stepwise fed-batch operations. Results obtained from the three different modes of operations can be predicted if the effects of both the concentration dilution and the agitation intensity change are considered.