When sulphides or mercaptans in glacial acetic acid solution are heated with permanganate, the resulting sulphonic acid compounds exhibit great similarity to compounds containing free carboxyl groups. The sulphonic acid group may also be directly introduced either by concentrated, or by fuming sulphuric acid, or by elimination of halogen by the action of sodium or silver sulphite on the halogen derivatives of the aliphatic compounds. Saturated hydrocarbons do not react with sulphur trioxide, but unsaturated hydrocarbons are readily attacked by SO_3. Similarly, halogenated compounds and alcohols react with concentrated or fuming sulphuric acid forming sulphonic and hydrosulphonic acids respectively. The aromatic compounds form, as a rule, sulphonic acids with much greater facility. Benzene, for instance, is easily converted into the _m_-disulphonic acid by gently heating with fuming sulphuric acid; stronger heating converts the _m_- into the _p_-disulphonic acid, and at 190° C. the trisulphonic acid is formed. Toluene treated with fuming sulphuric acid first yields _o_- and _p_-sulphonic acids, finally _o_- and _p_-disulphonic acids, ethylbenzene at the boiling point _p_-ethylbenzene-sulphonic acid. Of the three isomeric xylenes _o_- and _m_-xylene dissolve in concentrated, _p_-xylene in fuming sulphuric acid only. The action of sulphuric acid on naphthalene is stronger even than on benzene. Equal parts of naphthalene and sulphuric acid heated to 100° C. yield 80 per cent. [Greek: a] and 20 per cent. [Greek: b]-monosulphonic acid. At 160°-170°C. 25 per cent [Greek: a]- and 75 per cent. [Greek: b]-sulphonic acid is formed, and at higher temperatures [Greek: b]-monosulphonic acid only. If, on the other hand, 8 parts of naphthalene are heated with 3 parts of concentrated sulphuric acid to 180° C., two different naphthyldisulphonic acids are obtained. Complete solution of the substance in sulphuric acid is, generally speaking, a criterion of complete sulphonation. A completely sulphonated compound should remain clear on dilution with water, or, in case precipitation occurs, the precipitate should be completely soluble in alkali or ammonia. It is necessary to submit the product to this test, since many organic substances are soluble in concentrated sulphuric acid without undergoing any alteration in composition. Phosphoruspentoxide or potassium sulphate considerably increase the sulphonating property exhibited by fuming sulphuric acid. The separation of the sulphonic acids from sulphuric acid is effected by salting out the former with common salt, or by removing the sulphuric acid with calcium, barium, or lead salts, provided that the sulphonic acid salts of these metals are soluble in water.