SrFeyCo1-yOx (y = 0.0, 0.25, 0.50, 0.75, 0.90 and 1.0) thin films on sapphire substrates were prepared by laser deposition, and characterized by elemental analysis using Rutherford backscattering spectroscopy, particle-induced X-ray emission and inductively coupled plasma-atomic emission spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy. In order to determine the relative stability of the films at 500 degreesC, the films were subjected to both thermochemical oxidizing and reducing treatments in 100% O-2 and 2% H-2/argon gas mixtures, respectively. Thermochemical oxidizing treatments of the SrFeyCo1-yOx films resulted in dark-colored oxygen-rich films. Thermochemical reduction resulted in transparent yellow-brown films for the more iron-rich films, but not for the Co-rich films, which remained dark-colored. Fe-rich SrFeyCo1-yOx (y 0.50, 0.75 and 1.0) films exhibited the greatest degree of structural variation, resulting from the cubic perovskite brownmillerite phase change upon reduction. Films with higher Co substitution showed smaller variations in the unit cell parameters. The SrCoOx film was unstable under reducing conditions, with a Sr-enriched carbonate layer forming at the interface after exposure to ambient atmosphere. XRD measurements conducted in situ at 500 degreesC in both air and nitrogen gases confirmed that reversible structural changes occur in the film solely as a consequence of changing the surrounding gas composition, with the largest changes in lattice spacing occurring for the SrFe0.75Co0.25Ox and SrFe0.5C0.5Ox films. Crown Copyright (C) 2003 Published by Elsevier Science B.V. All rights reserved.