The oxidation of calcium sulphite in aqueous slurries of residual product from spray absorption of sulphur dioxide was studied under heterogeneous conditions typical of wet scrubbers. The observed rate of oxidation without additional catalyst was found to be of 0th and 1st order in the sulphite concentration and the partial pressure of oxygen, respectively. With manganeous sulphate added as a catalyst, the order was about 0.5 and 1.0, respectively. Both the residual product and the oxidation product were analysed with respect to composition and characteristics. The oxidation product contained more than 90 wt% gypsum.