A nearly single-phase theta-Al2O3 scale has been grown on two FeCrAl alloys, according to a time-temperature-transformation relationship far the formation of external alumina scales. SIMS sputter depth profiles of the O-18/O-16 distribution consistently indicate that theta-Al2O3 scales grow primarily by the outward transport of Al. This mechanism is consistent with the observed scale microstructure and is significantly different from tracer profiles for alpha-Al2O3 scales.