The mechanism of the vapour-phase mononitration of aromatics with dinitrogen tetroxide has been investigated over silica-alumina and the large-pore zeolite beta (BEA). A Rideal-Eley rate law is observed in both cases, showing that the reaction occurs between the chemisorbed aromatic and the gaseous nitrating agent. The effects of substituents are better correlated by a relation between the activity and the ionisation potential of the aromatic than by the Hammett equation. Both results conflict with the "classical" electrophilic aromatic substitution (SEAr), but agree with the formation of an aromatic radical cation intermediate at the surface of the catalyst and its reaction with a nitrogen dioxide radical in the rate determining step. The generation of electron-acceptor sites on the catalyst surface by nitrogen dioxide radicals is suggested. Addition of water in the feed inhibits the reaction by competitive adsorption over silica-alumina, but improves the efficiency of BEA, certainly by "steam cleaning" of the surface of this hydrophobic catalyst.