Nitrogen tetroxide (or Dinitrogen tetroxide) (N₂O₄) is a hypergolic propellant often used in combination with a hydrazine-based rocket fuel. The combination was used to fuel the Titan rockets used in the Gemini missions, and is still used today in the second stage engines of Delta II rockets.

Nitrogen dioxide is made by the catalytic oxidation of ammonia: steam is used as a diluent to reduce the combustion temperature. Most of the water is condensed out, and the gases are further cooled; the nitric oxide which was produced is oxidised to nitrogen dioxide, and the remainder of the water is removed as nitric acid. The gas is essentially pure nitrogen tetroxide, which is condensed in a brine-cooled liquefier.

Nitrogen tetroxide is a brownish yellow liquid which is easily vaporized. It is a powerful oxidizer, and is highly toxic and corrosive. However, it is not affected by mechanical shock and does not react with air. Dinitrogen tetroxide is always in equalibrium with Nitrogen_Dioxide (NO₂) and some Nitrogen Dioxide will be present in any quntity of Nitrogen tetroxide, higher temperatures push the equilibrium towards Nitrogen Dioxide.