A comparative study of O-2 reduction on well defined iron oxides Fe3O4 and gamma-FeOOH is presented. The Fe(III)/Fe(II) surface stoichiometry strongly depends on potential and determines the electronic conductivity and electron transfer rate to soluble redox species. A common feature of O-2 reduction on both oxides is the occurrence of Fe2+ surface sites simultaneous to O-2 reduction, which is necessary for the specific interaction between Fe(II) and O-2. The electrocatalytic differences for gamma-FeOOH and Fe3O4 are related to differences in oxide structure and different structure of the oxide-electrolyte interface.