Wet limestone scrubbing is one of the most widely used flue gas desulfurization processes for control of sulfur dioxide emissions from combustion of fossil fuels. A key step in the process is forced oxidation in the scrubber loop, which substantially improves the dewatering properties of the sludge, leading to the formation of gypsum (CaSO4 center dot 2H(2)O). This paper deals with the experimental study of calcium bisulfite oxidation in the presence of catalysts (ferrous and cuprous ions) added to the reaction vessel. A laboratory-scale apparatus was used; the experiments were performed at a fixed oxygen partial pressure (21.3 kPa) and at a temperature of 45 C. In particular, the effect of the simultaneous addition of both catalysts has been studied. The analysis of the experimental results, carried out by using the theory of mass transfer with a chemical reaction, indicates that the slow reaction regime has been explored, and the transition from the kinetic to the diffusional sub-regime has been reached. Experimental results have been compared with those obtained in the presence of the single catalytic species (Cu+ alone and Fe2+ alone), showing that copper is less effective than iron as a catalyst: no synergy between the two catalysts added simultaneously has been observed. (C) 2008 Elsevier B.V. All rights reserved.