At Mace Head, Eire, in the coastal East Atlantic, diiodomethane has been identified as an important precursor of iodine oxide radicals. Peak concentrations of both CH2I2 and IO at low water indicate that the intertidal region is a strong source of organo-iodines. Atmospheric measurements of CH2I2 made in marine air are compared with the concentrations predicted by a 2-dimensional model incorporating horizontal and vertical dispersion of surface emissions. The model shows that micrometeorological variability, proximity of the site to emissions, and photolysis all play important roles in determining the CH2I 2 concentrations at Mace Head. In addition to a tidal-height dependent intertidal flux, which was estimated from seaweed production data, a contribution from offshore (non-local) sources was required in order to reproduce the strong signature of photolysis in the CH2I2 observations. A combination of an offshore flux and an intertidal flux (of up to 1.4 × 109 molecules cm-2 s-1 at low water) results in good agreement between the measured and modelled CH 2I2 concentrations. Although this study does not necessarily infer emission of CH2I2 from the open ocean, it suggests that air-sea exchange of CH2I2 in coastal waters does occur.