Abstract: Sulfur oxides are removed from a waste gas containing SO.sub.2 by: scrubbing the gas with an aqueous slurry containing Mg(OH).sub.2 as well as gypsum thereby to transform the SO.sub.2 into MgSO.sub.3 and to fix the same in this slurry; oxidizing the MgSO.sub.3 in this slurry to convert the same into MgSO.sub.4 ; filtering the slurry containing an aqueous solution of the MgSO.sub.4 thus formed and gypsum thereby to recover the gypsum; and adding Ca(OH).sub.2 to the aqueous solution of the MgSO.sub.4 of the filtrate thereby to convert, by double decomposition reaction, at least the greater part of the MgSO.sub.4 into Mg(OH).sub.2 and, at the same time, the Ca(OH).sub.2 into gypsum, the resulting aqueous slurry containing the Mg(OH).sub.2 thus formed as well as the gypsum being used for the above mentioned scrubbing of the waste gas. The double decomposition reaction in this process is carried out in the presence of seed crystals of gypsum to obtain gypsum of good quality.