A type of chemical reaction in which a –SO3H group is substituted on a benzene ring to form a sulphonic acid. The reaction is carried out by refluxing with concentrated sulphuric(VI) acid for a long period. It can also occur with cold disulphuric(VI) acid (H2S2O7). Sulphonation is an example of electrophilic substitution in which the electrophile is a sulphur trioxide molecule, SO3.