Abstract: A process for preparing an over-based, oil-soluble magnesium salt of a sulphonic acid comprises contacting an acidic gas in the presence of a promoter system with a mixture of an oil-soluble magnesium salt of a sulphonic acid, a light magnesium oxide and an inert diluent. The promoter system comprises (1) a carboxylic compound selected from the group of compounds consisting of lower carboxylic acids, lower carboxylic anhydrides, substituted lower carboxylic acids, and metal salts and esters of lower carboxylic acids, (2) water, and optionally (3) a lower alkanol or lower alkoxy alkanol. The reaction is carried out at a temperature ranging from approximately 50.degree. F up to reflux temperature of the mixture. The volatile components are stripped from the reaction mixture after absorption of the acidic gas is at a desired level to give an over-based, oil-soluble magnesium salt of the sulphonic acid.