Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:

In atmospheric chemistry, air pollution, and related fields, nitrogen oxides refers specifically to NO_x (NO and NO₂).

Only the first three of these compounds can be isolated at room temperature. N₂O₃, N₂O₄, and N₂O₅ all decompose rapidly at room temperature. NO₃, N₄O, and N(NO₂)₃ are very reactive.

N₂O is stable and rather unreactive at room temperature, while NO and NO₂ are quite reactive but nevertheless quite stable when isolated.

NO_x (often written NOx) refers to NO and NO₂. They are produced during combustion, especially at high temperature. These two chemicals are important trace species in Earth's atmosphere. In the troposphere, during daylight, NO reacts with partly oxidized organic species (or the peroxy radical) to form NO₂, which is then photolyzed by sunlight to reform NO:

The oxygen atom formed in the second reaction then goes on to form ozone; this series of reactions is the main source of tropospheric ozone. CH₃O₂ is just one example of many partly oxidized organic molecules that can react with NO to form NO₂.