Dinitrogen trioxide is the chemical compound with the formula N₂O₃. This deep blue solid is one of the binary nitrogen oxides. It forms upon mixing equal parts of nitric oxide and nitrogen dioxide and cooling the mixture below -21 °C (-6 °F):

NO + NO₂ N₂O₃

Dinitrogen trioxide is only isolable at low temperatures, i.e. in the liquid and solid phases. At higher temperatures the equilibrium favors the constituent gases, with K_diss = 193 kPa (25 °C).