We report observations of iodine dioxide (OIO) in the remote marine boundary layer. The measurements were made at the remote site of Cape Grim in Tasmania, using the technique of differential optical absorption spectroscopy (DOAS), in the 540--570 nm spectral region. Employing a cross-section of 6.8¿10-17 cm2 at 548.6 nm, the concentration of OIO was found to vary from below the detection limit of the instrument (≈0.5 parts per trillion (ppt)) to a maximum of almost 3 ppt after sunset. During the day the upper limit to the atmospheric turnover time of OIO was found to be 20 min., when removal by gas-phase reaction and/or photolysis appears to dominate. At night the much longer turnover time of ≈1--4 hours can be explained by uptake on aerosol. ¿ 2001 American Geophysical Union