Abstract: Process of purification of magnesic raw material. Sulphur dioxide is allowed to react with an aqueous suspension of calcinated magnesite, crude or contaminated magnesium oxide, magnesium hydroxide, hydrated magnesium carbonates, or crude crystals of magnesium sulphite, and, after reaching a pH of 3 to 7.5 the reaction mixture is separated into a solid phase at least one fraction of which is rich in MgSO.sub.3.xH.sub.2 O, wherein x stands for 3 or 6, and at least one fraction containing concentrated impurities. The whole process is carried out at least once. A reaction mixture having a pH below 5 can be preferably transferred to pH 5.2 by the addition of an alkali, e.g. magnesium oxide or magnesium hydroxide, whereupon separation of at least two fractions of the solid phase is accomplished. The fractions obtained can be further purified by mixing in a hydrophilic liquid, and by a following separation. The fraction of crude magnesium sulphite crystals can be heated to a temperature over 60 degress C.