Abstract: Sulfur-containing fossil fuels are combusted in the presence of calcium carbonate or calcium magnesium carbonate, whereby the sulfur oxides formed by combustion react with the calcium oxide or calcium magnesium formed from the carbonate during the combustion, thereby to form calcium sulfate or calcium magnesium sulfate. The sulfur oxide content of the combustion gas is thereby reduced. The calcium oxide or calcium magnesium oxide is removed from the combustion, and is slaked with water to form the corresponding hydroxide. This hydroxide is recycled to the combustion zone or to the combustion gases emanating from the combustion zone, whereby the quantities of the carbonate used in the combustion can be reduced.