A colourless to pale yellow liquid or a brown gas, N₂O₄; r.d. 1.45 (liquid); m.p. –11.2°C; b.p. 21.2°C. It dissolves in water with reaction to give a mixture of nitric acid and nitrous acid. It may be readily prepared in the laboratory by the reaction of copper with concentrated nitric acid; mixed nitrogen oxides containing dinitrogen oxide may also be produced by heating metal nitrates. The solid compound is wholly N₂O₄ and the liquid is about 99% N₂O₄ at the boiling point; N₂O₄ is diamagnetic. In the gas phase it dissociates to give nitrogen dioxide

Because of the unpaired electron this is paramagnetic and brown. Liquid N₂O₄ has been widely studied as a nonaqueous solvent system (self-ionizes to NO⁺ and NO₃⁻). Dinitrogen tetroxide, along with other nitrogen oxides, is a product of combustion engines and is thought to be involved in the depletion of stratospheric ozone.