1,2-Dichlorobenzene can be reacted very selectively under adiabatic reaction conditions to give 3,4-dichloronitrobenzene by intensively mixing 1,2-dichlorobenzene with nitric acid, sulphuric acid and water, simultaneously or successively, in their total amount, using a mixing energy of 1-50 watts per litre of the total reaction mixture, preferably 3-30 W/l, and maintaining a temperature of from 0 to 60° C. during mixing.