The removal of SO2 by NH3 gas injection at various temperatures and moisture contents has been studied experimentally. The product compositions of the NH3-SO2-H2O vapor reactions were also reported. A thermodynamic analysis was carried out to predict the SO2 removal as well as the product compositions. Both the experimental results and thermodynamic analysis indicated that SO2 removal and the product compositions are sensitive to the reaction temperature. Moisture content, once in large excess of the stoichiometric requirement, does not have a strong effect on the product compositions but plays an important role in the SO2 removal.