Continuous, two phase sulphonation of monocyclic aromatic hydrocarbons was investigated. The reactions were carried out in a rotation-film reactor characterized by excellent heat and mass transfer properties. In the experiments benzene was sulphonated by sulphuric acid and oleum. The results of these experiments are discussed. A reverse phase high pressure liquid chromatographic method was elaborated for the determination of the sulphonic acid isomers and the diphenylsulphon sulphonic acid type by-products and, in the knowledge of their amounts, the sulphonation reaction could be followed. The experiments were carried out both at atmospheric and reduced pressures. The mono- and disulphonic acids produced in the sulphonation of benzene are important raw materials in the production of dyestuffs and pharmaceuticals. Considerable amounts of these compounds are used as intermediates in the production of wetting agents, ion exchangers and pickling liquors. Benzenesulphonic acids are effective catalysts.