ClOOCl was prepared in situ in two static reactors and in a flow tube by photolyzing OClO or by the reaction of Cl with O3, at temperatures ≤253 K. IR and UV spectra were monitored simultaneously, and concentrations of ClOOCl were determined from the known UV absorption cross sections. Based on these concentrations, absorption coefficients and integrated band intensities of the &ngr;5 and &ngr;1 bands of ClOOCl near 650 cm-1 and 750 cm-1 were derived for spectral resolutions between 1 and 0.125 cm-1 using a FTIR spectrometer. Diode laser spectra of several regions of the &ngr;5 band demonstrate that the rotational structure of this band can be resolved at total pressures of several mbar. The low line intensities of the 650 cm-1 band, however, do not allow to detect ClOOCl by airborne diode laser absorption at today's detection sensitivities.¿ 1997 American Geophysical Union