Photochemical redox cycling of iron in natural seawater may affect the availability of iron to phytoplankton for cellular growth. Fe(III) colloids that support only limited phytoplankton growth were added to coastal seawater at pH8 and exposed to simulated solar irradiation for varied time intervals. Photo-production of Fe(II) in the seawater samples was measured during each irradiation. Following irradiation, the marine diatom, Skeletonema costatum, was inoculated into each treatment and cell densities were monitored in batch cultures over the next ten days. Increases in steady state concentrations of Fe(II) were observed during irradiation and subsequent elevated phytoplankton growth was observed in most solutions exposed for 60 minutes or more. The photo-production of Fe(II) in seawater results in the transfer of iron between chemical states which differ in their ability to supply iron on a time scale required for phytoplankton growth.