A study on the oxidation of calcium sulfite catalyzed by Mn2+ is presented in this paper. A laboratory-scale well-mixed thermostated reactor was used, with continuous feeding of both gas and liquid phase. The gas phase was a mixture of oxygen and nitrogen, with O-2 partial pressure varying from 5.1 to 40.5 kPa. The liquid phase was obtained by mixing inside the reactor two separate aqueous solutions, one obtained by dissolving Ca(OH)(2) into a SO2 solution, and the other one containing manganous sulfate. The temperature was kept constant in all the experiments at 45 degrees C. The results show that increasing catalyst concentration, the transition takes place from slow kinetic to slow diffusional and eventually to fast reaction regime.