The photochemistry of iron in surface waters, and its implications to iron bioavailability, was examined on two cruises to the equatorial Pacific. Decktop incubations were performed with equatorial seawater to which iron was added in various chemical forms. Results showed clear diurnal patterns in measurable iron levels, with the highest levels occurring midday. Model calculations were based on independently determined rate constants. The authors suggest that photochemical reactions may have a significant impact on iron availability to phytoplankton in the open ocean.