Liquid benzene was sulphonated with gaseous sulphur trioxide in a tube reactor and in a new gas—liquid cyclone reactor. The products are benzenesulphonic acid and diphenyl sulphone (byproduct). The observed selectivity depends on the conversion, the initial benzene concentration and the mass transfer characteristics of the reactor. Minimum diphenyl sulphone formation was obtained for a low initial benzene concentration, a low benzene conversion and with a high liquid-side mass transfer coefficient, as in the cyclone reactor. As the actual mass transfer rate during sulphonation could not be measured, the observed selectivity was related to the mass transfer coefficient determined by the simultaneous absorption of CO2and O2 in aqueous sodium hydroxide solution.