The oxidation behaviour and the subsequent chemical failure of a Fe-20Cr-5Al based alloy were studied at 1200°C in laboratory air, at times of up to 700 hours. Tests on 70 micron thick foils, showed void formation at the metal/oxide interface soon after the aluminium content in the alloy dropped below a critical level (≤0.5 wt%). At this stage, the alloy could no longer sustain alumina scale formation and resulted in the initiation and development of a Cr-rich sub-layer. This chromia layer was found to be continuous and of a uniform thickness. As the sub-layer formed, voids were also observed at the metal/oxide interface. The voids were found to fill with chromia after further exposure. It is thought that the change in oxide growth mechanism from alumina to chromia growth is responsible for the void formation. This also explains the lack of void formation during the sustainable growth of the alumina scale. The introduction of silicon to the Fe-20Cr-5Al based alloy via a diffusion couple was found to significantly influence the oxidation behaviour of the thin foils. Void formation was observed directly beneath the alumina scale and filled voids were now found to contain silicon oxide rather than chromia. The void filling mechanism also appeared to be different. With chromia filled voids, the filling commenced from the underside of the oxide, with the oxide growing inwards, while silica containing voids were filled by silica growing outwards into the void from the substrate. Throughout the study, optical and scanning electron microscopes were used to analyse all stages of oxidation and the subsequent failure of the thin foil samples. EBSD was also used to generate a more comprehensive analysis of selected locations.