This paper analyses the degradation of a ceramic top coating 70%ZrO2 – 30%CaO deposited onto a stainless steel AISI 304 by thermal spray, using Ni-6%Al-5%Mo as overlay coating. These thermal barrier coatings were heat treated for 48, 120 and 288 h at 800 °C to evaluate the degradation of these materials by isothermal oxidation. The microstructure evolution during oxidation was analysed by environmental scanning electron microscopy, transmission electron microscopy and X ray microanalysis. A thermally grown oxide layer was observed between the overlay coating and the ceramic top coating after oxidation. This layer was formed by a mixed Al, Ni and Mo oxides.