Removal of SO2 from flue gas by the absorbent prepared from coal fly ash, calcium oxide, and calcium sulfate was studied under different reaction conditions to elucidate the effects of the reaction temperature, water vapor pressure, and coexistence of NO in a flue gas. The SO2 removal activity increased with an increase in NO concentration up to 500 ppm at 130 degrees C. The SO2 removal activity increased as the water vapor pressure increased until a monolayer coverage with water molecule is achieved. As the adsorbed water exceeded the monolayer coverage, the SO2 removal activity suddenly decreased and calcium sulfite became the main product instead of calcium sulfate. The NO removal activity increased with an increase in SO2 concentration up to 2000 ppm at 130 degrees C. The NO removal also increased with an increase in water vapor pressure. The NO removal markedly decreased as the adsorbed water exceeded the monolayer coverage.