A recent re-examination of the kinetics of the reaction of NO2 with HO2 shows this to be a three body reaction leading to the formation of peroxynitric acid (HO2NO2). The rate of formation is such that significant concentrations of HO2NO2 (not previously included in atmospheric models) are expected to form in the stratosphere. In the present study a preliminary determination of the absorption cross section of HO2NO2 is presented. This information is used to calculate theoretical vertical concentration profiles for HO2NO2. It is concluded that peak mixing ratios should be in the range of 1--3 ppb at 30 km, and that it should be possible to detect this new stratospheric species using infrared techniques. ¿ 1977 American Geophysical Union