Aromatic diisocyanates are produced by reacting in the gas phase the corresponding primary aromatic diamine with phosgene. The phosgene and the primary aromatic diamine are reacted within a mean residence time of from 0.05 to 15 seconds. The aromatic diamine used contains less than 0.05 mole % overall of aliphatic amine containing no keto groups, per mole of primary amino groups.