Previously published band strengths of the ClO fundamental (800--880 cm-1) show large uncertainties and inconsistencies, mainly caused by the problem of accurate particle density determination of the instable radical ClO. A new procedure has been elaborated, utilizing additional far-infrared (FIR) measurements of pure rotational transitions. ClO was prepared in a flow reaction of dichlorine monoxide with atomic chlorine and measured in a 72 m path White-type absorption cell. A series of subsequent FIR, midinfrared (MIR) and FIR spectra was recorded with a Bruker IFS 120 HR Fourier transform spectrometer within the same experiment. From the FIR transitions and their calculated line strengths the particle density could be determined. Drifts of the ClO concentration during the flow experiment were monitored by the analysis of both FIR spectra bracketing the MIR measurement. The line strengths of 72 transitions in the 2&Pgr;3/2 state were evaluated allowing the determination of the vibrational transition dipole moment (-0.03224(53)D) as well as the linear (+0.00684(80)) and quadratic (+1.56(64)¿10-4) Herman-Wallis coefficients (1&sgr; uncertainties given).¿ 1997 American Geophysical Union