A process is described for preparing a water-soluble sulphonated dispersant from a petroleum hydrocracking residue comprising: contacting a petroleum hydrocracking residue boiling above 524.degree. C. with concentrated sulphuric acid, oleum or sulphur trioxide to form a sulphonated mixture containing water soluble and water insoluble products, and separating the water soluble sulphonated product from the mixture. The mixture of water soluble and water insoluble sulphonated products is neutralized with alkali and evaporated to dryness, after which the dried mixture is extracted with alcohol to leave a water soluble product. The product can be used as a water reducer or superplasticizer in concrete.