A bulk sulfonation process for the preparation of sulfonated polymers from hydrocarbon polymers having olefinic unsaturation or aromatic moieties, or both, includes intimately mixing the polymer by mechanical means in the absence of a solvent for the polymers with a sulfonation reagent at a sufficient temperature and time to effect the desired degree of sulfonation. The sulfonated polymer may be recovered as the free sulfonic acid or neutralized ionically with a basic compound. Sulfonated polymers including a substantial proportion of aromatic moieties yield water-soluble or water-dispersible salts, useful as thickening and flocculating agents, when neutralized with ammonia monobasic cations or lower alkyl amines. These materials, however, when sulfonated at a low level, e.g.