OClO and NO2 slant column abundances have been measured by ground-based spectrographs in both the Arctic (Kangerlussuaq, Greenland, at 67.0 ¿N, 51.0 ¿W) and the Antarctic (McMurdo Station, Antarctica, at 77.8 ¿S, 166.6 ¿E). A key process for stratospheric ozone depletion is the activation of chlorine by heterogeneous reactions. Sunlight is also an essential ingredient in the catalytic cycles that destroy ozone. This data set illustrates the seasonal timing of autumnal chlorine activation and the springtime disappearance of active chlorine at two polar sites. As such, the data provide constraints for model calculations. In the Northern Hemisphere during the winters of 1994--1995 and 1995--1996, the OClO data show the seasonal onset of stratospheric chlorine activation in mid-December and seasonal disappearance of active chlorine in mid-March. The data also show the seasonal decline and recovery (in mid-March) of nitrogen dioxide levels. In the Southern Hemisphere during the winters of 1991, 1993, and 1996, similar chemical processing occurs during late April to early May and late September to early October. The role of several factors in determining the seasonal cycles of column OClO are probed through model studies. Observed cold temperatures and reactions on liquid sulfate aerosols appear to play a key role in producing the timing of the observed OClO seasonal cycle in the Northern Hemisphere. ¿ 1999 American Geophysical Union