We report measurements of the iodine monoxide (IO) radical in the marine boundary layer at three remote sites: Mace Head (Ireland), Tenerife (Canary Islands), and Cape Grim (Tasmania). IO was observed by long-path differential optical absorption spectroscopy using the A2&Pgr;3/2-X2&Pgr;3/2 electronic transition between 415 and 450 nm. The daytime IO concentration at these three locations was found to vary from below the detection limit (≤ 0.2 parts per trillion (ppt) to a maximum of 4 ppt, with an average of about 1 ppt. Of particular note is that the IO observed off the north coast of Tenerife, which is probably typical of the open ocean sub-tropical North Atlantic, exhibited a distinct diurnal cycle which correlated strongly with the solar actinic flux in the near UV. IO was also observed at Cape Grim to be present at much lower levels (≈ 0.3 ppt) in westerly air from the Southern Ocean. As is shown in the companion paper (McFiggans et al., this issue), these measurements of IO are satisfactorily reproduced by a photochemical box model incorporating the recycling of iodine through marine aerosol. This model indicates that the direct iodine-catalyzed destruction of ozone in the boundary layer may well be similar to the losses caused by odd-hydrogen photochemistry and dry deposition. The significance of this work is that IO is probably present in much of the open ocean boundary layer, at levels where it may cause significant depletion of ozone. ¿ 2000 American Geophysical Union