The quartz crystal oscillator microbalance has been used in conjunction with Auger electron and x-ray photoelectron spectroscopies to study the oxidation at 185°C and 1 atm O2, of air-exposed Permalloy films vacuum deposited at 200° and 350°C. Frequency shifts corresponding to weight gains of the films during oxidation combined with data obtained from AES, XPS, and ion sputter-etching showed: (i) the oxidation rate was diffusion controlled, (ii) Fe was preferentially oxidized with moderate Ni enrichment in the remaining alloy, and (iii) the average iron oxide composition is FeO1.3. Reaction appears to be limited by iron diffusion through the alloy. Average iron diffusion coefficients estimated from the parabolic rate constants are in good agreement with that reported for iron in thin polycrystalline Ni films. © 1977, The Electrochemical Society, Inc. All rights reserved.