The absorbents for SO2 and NO from flue gas were prepared from calcium hydroxide, calcium sulfate, silicic acid, and aluminum hydroxide. The effects of the composition of the absorbent are studied on the activity for the absorption of SO2 and NO and on the structure of the absorbent. The activity for the absorption of SO2 and NO markedly increased with the content of silica in the absorbent up to 40%. The formation of calcium silicate is suggested to be predominant in a high concentration of silica, while the formation of ettringite was observed by the XRD only for the absorbent containing silica below 30%. The distribution of the sulfur and nitrogen compounds in the absorbent revealed by XPS suggests that adsorbed nitrogen compounds are gradually replaced by sulfur compounds as the reaction proceeds.