Photochemical reactions among chlorine, nitrogen dioxide, and formaldehyde were studied, using parts-per-million concentrations in 1 atm of air. The reactant mixtures were irradiated by ultraviolet fluorescent lamps and simultaneously analyzed by the Fourier transform infrared technique by use of folded light paths up to 504m. With an excess of NO2 over Cl2, the reaction products included O3, CO, HNO3, N2O5, HCl, and nitryl chloride (ClNO2). When chlorine exceeded NO2, the principal product was peroxy nitric acid (HOONO2). Peroxy formyl nitrate, nitrous acid, and chlorine nitrate were not seen. The nitryl chloride was stable even with the ultraviolet lights on. The peroxy nitric acid disappeared from the cell with a half-life of about 10 min. Formyl radicals (HCO), unlike acetyl radicals, did not combine with O2 and NO2 by addition. HCO reacted with O2 to yield HOONO2. If NO is present, the HO2 will prefer to react with it, oxidizing it to NO2. (Copyright (c) 1977 by the American Chemical Society.)