Chlorine monoxide (ClO) measurements have been recorded since December 1993 using a ground-based radiometer built in the United States and installed at a European site belonging to the Network for the Detection of Stratospheric Change, namely the Plateau de Bure (45 ¿N, 10 ¿E, 2550 m altitude). This paper describes the installed instrument, details the calibration and data processing routines that have been developed for use with the instrument, and characterizes the optimum estimation method used to retrieve vertical profiles and the associated error budget. Great care is taken to lessen biases induced by instrumental baseline ripples. We show the best data obtained so far, recorded from January 3 to 5, 1995, during the Second European Stratospheric Arctic and Midlatitude Experiment (SESAME) Campaign. The diurnal variation of ClO measured from 25 to 50 km is presented and compared with results from a zero-dimensional and a three-dimensional model. The remainder of the data is still being processed, although a great deal of this data has been degraded by systematic instrumental artifacts in the spectral baseline.¿ 1997 American Geophysical Union