On the basis of physical and chemical equilibria conditions a model of gaseous sulfur dioxide solubility in the solutions applied in an FGD (flue-gas desulfurization) process was developed. The dependencies of solution pH in the bulk liquid and in the limestone particle vicinity on the calcium carbonate and/or on the dissolved sulfur dioxide content are presented. The influence of the concentration of dissolved carbon dioxide on pH, sulfite to sulfate oxidation conditions, limestone and calcium sulfite solubility are also discussed. The mechanisms leading to a decrease in calcium carbonate solubility or limestone particle blinding are explained. The restrictions in calcium chloride concentration for the reliable operation of an FGD process are predicted.