Culture experiments with the coastal marine diatom Thalassiosira pseudonana (WHOI clone 3H) demonstrate that, as an iron source, freshly prepared colloidal ferric hydroxide can produce better cell yield than the more crystalline goethite or hematite, Ageing or heating of the prepared ferric hydroxide stock causes a reduction in cell yield. This reduction appears to be related to increased thermodynamic stability of the colloid as suggested by thermogravimetric analysis and relative dissolution rates. The reduction in cell yield can be prevented by the addition of the chelating agent EDTA prior to, but not after, ageing or heating of the ferric hydroxide stock. These results suggest that the ability of colloidal iron to provide a source of metal for phytoplankton is related to the thermodynamic stability of the colloid.