Bacteria serve as important catalysts in many geochemical cycles, including the oxidation of Fe(II) in aquatic and terrestrial environments. As catalysts, bacteria influence the mobilization and precipitation of Fe. Although iron oxides are the most abundant metal oxide in soils and sediments, there has been no systematic laboratory study of the bacterial influences on the d¹⁸O and values of Fe(III)- oxides produced from Fe(II)-oxidizing bacteria. We are analyzing the d¹⁸O and d⁵⁶Fe values of iron oxides produced in an acid mine drainage field site and by a pure culture of neutrophilic Fe(II)-oxidizing bacteria. We have also analyzed the d¹⁸O values of synthetic ferrihydrite, a common, unstable intermediate of Fe(II)oxidation and Fe(III) mineralization. Although consistent d¹⁸O values have been measured from two different syntheses of ferrihydirite, the analytical precision of these measurements requires further improvement. The results of all of these experiments will be presented and their biogeochemical implications discussed.