The O-18(2) exposure-based approach was used to investigate the failure mechanisms of the oxide scales growing on alumina-forming materials. The scale spallation mechanisms and cracking processes were studied at various oxidation stages of FeCrAl alloys and beta-NiAl intermetallic compound. High spatial resolution SIMS was applied to determine the distribution of the oxygen isotopes and other elements in the scales. It was found that the scales spall away according to an adhesive mode. However, this process usually occurs at temperatures high enough to cause the reoxidation of the exposed bare substrate which results in thin oxide film on the metal. The thickness of this film and its composition depend on the alloy and the region at the interface. Spallation on reactive-element free FeCrAl alloys occurs at relatively high temperatures and the film is fairly thick, while on Zr-containing material very thin oxide layer is formed because the scale is better resistant to spalling and this process occurs at quite low temperature. The thin oxide layer formed on smooth regions comprises essentially the alumina, while the sequence of iron, chromium and aluminiurn oxide appears on regions exhibiting 'oxide imprints'. The applied approach enabled to find that the through-scale cracking observed at early oxidation stages of beta-NiAl occurs at high temperatures and not during cooling. Formation of such cracks affects the further growth of the scale in terms of its microstructure, morphology and generation of stresses. Oxygen inward penetration through cracked scales formed during thermal cycling of FeCrAl alloys occurred mainly via oxide grain boundaries.