Results are reported on the electronic disorder in the perovskite SrFeO3-Delta and in the spinel Fe3O4. Data have been obtained from high-temperature in-situ Mossbauer studies extending up to 1400 degreesC. The electronic disorder of the mixed valence iron perovskite SrFeO3-Delta has been studied as a function of temperature and oxygen partial pressure. Results are discussed in relation to the microscopic mechanism of electrical conduction. In magnetite, Fe3O4, the cation distribution of ferrous and ferric ions in the spinel structure has been determined as a function of temperature. (C) 2000 Elsevier Science B.V. All rights reserved.