ClONO2 was observed in high-resolution infrared emission spectra obtained on November 6, 1984, by a balloon-borne Fourier spectrometer. The observations took place near 0300 LT at a latitude of 35¿ N. Spectral simulations are used to determine the mixing ratios of ClONO2. This analysis incorporates line by line calculations and new ClONO2 cross sections measured in the laboratory at 223 K. The inferred mixing ratios of ClONO2 are 1.3¿0.45 ppb and 0.98¿0.35 ppb at 14 and 34 mbar. One-dimensional photochemical model predictions are compared to the observations. The ClONO2 mixing ratio at 34 mbar appears to be larger than theory, while there is agreement at 14 mbar.