Failure during thermal cycling in air of silicide and aluminide coatings on a niobium-base alloy has been investigated and failure mechanisms proposed. Most failures occurred at or very near geometrical transitions between the curved and flat sections of the specimen. In the neighborhood of these transitions, cracks form in the coating at oblique angles to the specimen surface due to non-uniform deformation induced by the thermal stress. These oblique cracks allow easy detachment of coating segments and exposure of the substrate to the oxidizing environment. Cracks in aluminide coatings were found to multiply much more rapidly during cycling than in suicide coatings. This leads to the development of numerous oblique cracks and consequently the protective life of aluminide coatings is well below that of silicide coatings.