KNOWLEDGE of the life cycle and budget of nitrogen oxides in the atmosphere is important for several reasons. In the stratosphere, NO and NO2 (NOX) play a dominant part in maintaining the structure and stability of the ozone distribution. In the troposphere, NOX is involved in the production and destruction of OH radicals which control the fluxes of many important species such as CH4, CO, H2 and CH3Cl, to the stratosphere. In polluted urban areas, NOX chemistry provides the basic formation mechanism of photochemical air pollution. In all these areas of atmospheric chemistry, the interactions of NOX with HOX species (HO, HO2 and so on) are of great significance, particularly the reactions of HO2 with NO and of HO with NO2. A further HOX–NOX interaction is the proposed1 formation of peroxynitric acid in reactions (1) and (−1): We present here a study of the extent to which reaction (1) and (−1) acts as a sink for HOX and NOX in the atmosphere.