Absorption by the Q branch of the &ngr;4 band of CIONO2 at 780.2 cm-1 has been tentatively identified in a series of 0.02-cm-1-resolution balloon-borne solar absorption spectra of the stratosphere. The spectral data were recorded at sunset from a float altitude of 33.5 km during a balloon flight from Holloman Air Force Base (32.8 ¿N, 106.0 ¿W) near Alamogordo, New Mexico, on March 23, 1981. A preliminary ClONO2 vertical profile has been determined from the stratospheric spectra by using the technique of nonlinear least squares spectral curve fitting and new spectroscopic parameters deduced from high-resolution laboratory spectra of ClONO2 and O3.