The first spectroscopic measurements of chlorine dioxide throughout Antarctic fall, winter, and spring were carried out at McMurdo Station during April to October 1991. Two different observing modes were employed to extend the measurements over the broadest possible seasonal range: direct Moon measurements were used as well as observations of the scattered light from the sky at an 80¿ angle relative to the zenith in the direction toward the Sun. The latter observing scheme facilitates measurements during the late twlight, when the brightness in the more conventional zenith viewing direction is much weaker. The measurements made using both the scattered light and the lunar sources show that OClO was below detection limits in late April and May (twight OClO vertical column <3¿1012 cm-2), for a corresponding total ozone loss rate due to the coupling of chlorine and bromine of no more than about 0.25 DU/d. OClO was first observed in late June using lunar spectra, at which time high nightime vertical column abundances of about 1.0¿1014 cm-2 were measured, implying extensive conversion of inorganic chlorine to reactive forms in the lower stratosphere. The observation of OClO in midwinter implies that the air parcels had been exposed to sunlight, presumably via wave-driven excursions of airflow to latitudes outside the polar night region. High levels of OClO were measured using moonlight near the full Moon phases in late July, August, and September. Further, these measurements of high OClO indicate that significant ozone loss should have taken place during midwinter. The winter ozone loss rate suggested by these data is likely to increase in the future as atmospheric loading of chlorine and bromine continues to rise. In addition, scattering light measurements in late July and August revealed very large twilight column abundances then, implying significant ozone loss rates. Somewhat smaller twilight column abundances were measured in September, and the OClO dropped below detection levels in October. ¿American Geophysical Union 1993