Abstract: Described herein is a process for the preparation of 3,4-diaminobenzenesulphonic acid, characterized in that 1,2-diaminobenzene is reacted with anhydrous sulphuric acid which optionally comprises SO3 in up to stoichiometric amount, at a temperature in the range from 100 to 160° C. with stirring for a reaction time of from 1 to 20 hours; water or ice is added to the reaction mixture, optionally with cooling, up to a sulphuric acid concentration in the range from 30 to 75% by weight, based on the total mixture; the 3,4-diaminobenzenesulphonic acid precipitated out of the reaction mixture is filtered off, optionally washed with dilute sulphuric acid and worked up.