An industrial desulphurization process is modelled on the basis of equilibrium data, mass balance, mass transfer and plant data. For the Ca2+-SO2-H2O system the equilibrium diagram at 25degreesC is constructed. The diagram presents the concentration of total SO2 combined SO2, true free SO2, the pH value and the solubility curve for CaSO3.1/2H(2)O. Besides the basic chemistry occuring in the process, the role of relevant additional chemical species is explained. The overall desulphurization process is studied through the analysis of the partial processes: absorption of SO2 and sulphite to bisulphite reaction: limestone dissolution and its reaction with Ca(HSO3)(2); absorption of O-2, oxidation of CaSO3 and gypsum crystallization mass balance and the mass transfer. The model is tested on the basis of data taken from an industrial plant.