Measurements of the concentrations of OH and HO2 (HOx) in the high-latitude lower stratosphere imply the existence of unknown photolytic sources of HOx. The strength of the additional HOx source required to match the observations depends only weakly on solar zenith angle (SZA) for 80¿<SZA<93¿. The wavelengths responsible for producing this HOx must be longer than 650 nm because the flux at shorter wavelengths is significantly attenuated at high SZA by scattering and absorption. Provided that the sources involve only a single photon, the strength of the bonds being broken must be <45 kcal mole-1. We speculate that peroxynitric acid (HNO4) dissociates after excitation to an unknown excited state with an integrated band cross section 2-3¿10-20 cm2 molecule-1 nm (650<&lgr;<1250 nm). ¿ 1999 American Geophysical Union