For planar solid oxide fuel cell (SOFC) designs chromia forming ferritic steels are being considered as possible construction materials for the interconnections. In accordance with SOFC market requirements it is in many cases necessary to reduce the size of the fuel cell and thus the thickness of the interconnect. Therefore, long term, cyclic oxidation studies of ferritic interconnector steels were carried out at and in air thereby putting main emphasis on the effect of specimen thickness on the oxidation behaviour. It was observed that with decreasing sample thickness the life time of the mentioned alloys decreases due to breakaway phenomena. This effect is caused by the smaller chromium reservoir in the alloy in case of thinner components. The observed life time limits can be predicted with reasonable accuracy by a theoretical model, using oxide growth rate parameters, initial alloy content and critical content for protective chromia scale formation. It also has to be taken into account that the oxidation rates of the steels increase with decreasing specimen thickness.