We report the first long-term analysis of the total column of ClONO2 from ground-based infrared solar absorption measurements recorded at Lauder, New Zealand <45¿ S, 170¿ E, 370 m above sea level (asl)>. Between September 1990 and August 1994 the ClONO2 total column averaged 1.22(¿0.4)¿1015 cm-2 (1&sgr; standard deviation), in good agreement with previous airborne and spaceborne measurements at southern hemisphere midlatitudes. The monthly mean ClONO2 column over Lauder shows a clear seasonal cycle with a peak amplitude of 22(¿3)% of the yearly mean (1&sgr; standard error) with the seasonal maximum occurring in early September. This is the first measured seasonal variation of ClONO2. Within the error limits, the derived September 1990 to August 1994 total column increase rate of 1.3(¿2)% per year (1&sgr; standard error) is consistent with determinations of the recent growth rate of atmospheric chlorine. ¿ American Geophysical Union 1995