Wet limestone scrubbing is the most common flue gas desulfurization process for control of sulfur dioxide emissions from combustion of fossil fuels. Forced oxidation in the scrubber loop improves the dewatering properties of the sludge, leading to the formation of gypsum (Ca50 . 2H(2)O). A literature analysis revealed that uncertainties on the mechanisms of the oxidation reaction and on the values of the kinetic parameters still remain. In the present work the oxidation rate was experimentally studied by contacting pure oxygen or mixtures of oxygen and nitrogen with a calcium bisulfite solution. The experiments were carried out in a well-mixed bubbling reactor varying temperature, oxygen partial pressure and sulfite concentration, in the absence of solid calcium sulfite and of catalytic species. It was shown that the rate of the process is controlled by reaction kinetics, and that the reaction rate is zero order in dissolved oxygen and 3/2-order in bisulfite ion.