First measurements of iodine oxide (IO) in the Antarctic troposphere are reported. Since March 1999, a newly developed dual channel spectrograph has been continuously performing Differential Optical Absorption Spectroscopy (DOAS) measurements of zenith scattered sunlight at Neumayer-Station, Antarctica (70¿39' S, 8¿15' W). The spectral signature of IO was clearly detected by observing five vibrational absorption bands located in the wavelength region between 415 and 461 nm. The observed diurnal variation of IO is characterized by a rapid decrease in the differential slant column density (DSCD) with increasing solar zenith angle (SZA) during twilight. This observation points to a fast conversion of reactive iodine into its nighttime reservoir species. It also strongly indicates that the detected IO is located in the troposphere. The decrease of the IO DSCD of up to 1⋅1014 molec/cm2 between 80¿ and 95¿ SZA is unexpectedly large. Under the assumption that IO is located in the marine boundary layer (MBL) (below 2 km), IO mixing ratios may reach up to ~10 ppt. The seasonal variation shows higher IO amounts during summer than during winter. This finding is possibly caused by the smaller distance to the open sea, where the iodocarbons are emitted, and by the more efficient photodissociation of the organic iodine precursors. ¿ 2001 American Geophysical Union