To investigate the effect of the ammonium concentration on SO2 absorption with a chemical reaction in the wet limestone-gypsum flue gas desulfurization (FGD) process, the SO2 absorption rate in limestone slurry was studied by using a stirred tank reactor at a constant pH, high temperature (50degreesC) and under the presence of ammonium. An absorption model with a reaction plane and the presence of ammonium in the liquid based on the film model was also proposed for an actual FGD plant. For the SO2 absorption rate depending on the concentration of ammonium, the experimental values agreed quite well with the results calculated with the absorption model, taking the presence or ammonium in the liquid into consideration.