A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne, which led from Tomako-mai, Japan (42 • N, 141 • E) through the Western Pacific to Townsville, Australia (19 • S, 146 • E) in October 2009. In the marine boundary layer within the tropics (between 20 • N and 5 • S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical sim-ulations indicate that the organic iodine precursors observed during the cruise (CH 3 I, CH 2 I 2 , CH 2 ClI, CH 2 BrI) are not sufficient to explain the measured IO mixing ratios. Reason-able agreement between measured and modelled IO can only be achieved if an additional sea-air flux of inorganic iodine (e.g., I 2) is assumed in the model. Our observations add fur-ther evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer.