Sulphonic acids, the calcium salts of which are soluble in water are obtained from those whose calcium salts are insoluble in water by treatment, if necessary at elevated temperatures, with sulphuric acid, chlorsulphonic acid or an analogously acting reagent. The viscous sulphonic acids obtained in the early stages of processes for refining mineral oils with sulphuric acid may thus be converted into hard sulphonic acids having soluble calcium salts. The latter may be used for making dispersions and emulsions, e.g., of asphalts, bitumen, paraffin, vegetable and animal oils and rubber. They may also be used for stabilizing dispersions e.g. against changes of temperature and the addition of electrolytes and for breaking emulsions of an aqueous phase in a non-aqueous phase such as mineral oil emulsions. In examples (1) acid tar obtained from edeleanized spindle oil is diluted with water and the viscous mass obtained heated with sulphuric acid. On dilution, hard sulphonic acids having soluble calcium salts are precipitated; (2) acid tar obtained from the spindle oil distillate is directly heated with oleum; (3) the acid tar used in example 1, is converted into sulphonic acids having soluble calcium salts by heating with an acid tar obtained from edeleanized spindle oil by treatment with oleum; (4) the tar acid used in examples 1 or 2 is diluted with water, and the viscous mass still containing sulphuric acid is heated for some days; (5) a sulphur dioxide extract of a spindle oil distillate is treated with oleum, and the acid tar obtained is heated with sulphuric acid. Acid tars obtained from hydrocarbons previously subjected to a cracking treatment or to hydrogenation may be similarly converted to sulphonic acids having soluble calcium salts.ALSO:Sulphonic acids, the calcium salts of which are soluble in water, are obtained from those whose calcium salts are insoluble in water by treatment, if necessary at elevated temperatures, with sulphuric acid, chlorsulphonic acid or an analogously acting reagent. The viscous sulphonic acids obtained in the early stages of processes for refining mineral oils with sulphuric acid may thus be converted into hard sulphonic acids having soluble calcium salts. The latter may be used as wetting agents and for making dispersions and emulsions, e.g. of asphalts, bitumen, paraffin, vegetable and animal oils, and rubber. They may also be used for stabilizing dispersions, e.g. against changes of temperature and the addition of electrolytes. In examples: (1) acid tar obtained from edeleanized spindle oil is diluted with water and the viscous mass obtained heated with sulphuric acid. On dilution, hard sulphonic acids having soluble calcium salts are precipitated: (2) acid tar obtained from a spindle oil distillate is directly heated with oleum; (3) the acid tar used in example (1) is converted into sulphonic acids having soluble calcium salts by heating with an acid tar obtained from edeleanized spindle oil by treatment with oleum; (4) the acid tar used in examples (1) or (2) is diluted with water, and the viscous mass still containing sulphuric acid is heated for some days ; (5) a sulphurdioxide extract of a spindle oil distillate is treated with oleum, and the acid tar obtained is heated with sulphuric acid. Acid tars obtained from hydrocarbons previously subjected to a cracking treatment or to hydrogenation may be similarly converted to sulphonic acids having soluble calcium salts. The products may be used for breaking emulsions consisting of an aqueous phase in a non-aqueous phase.