ALTHOUGH liquid water is a minor component of the troposphere, the chemical reactions that occur in water droplets (clouds, fog or rain) affect the composition of the atmosphere and the atmos-pheric input to terrestrial and aquatic ecosystems. Iron (Fe(II) and Fe(III)) is involved in many of these processes, particularly redox and radical chain reactions16. Here we report measurements of Fe(II) concentration (up to 200 μ?) in fog water (pH 37). We find that a large fraction of the total Fe is present as dissolved Fe(II). The concentrations of Fe(II) increase both with exposure to light and with decreasing pH. The concentrations of dissolved iron in fog droplets are much higher than those measured in oxic surface waters and so atmospheric deposition may be a source of dissolved Fe(II) to surface waters, thereby increasing the availability of iron to aquatic biota.