Thin spectrally selective solar absorber coatings have to be prevented from degradation caused by the substrate acting as a diffusion source. The effect of an intermediate layer sputtered in between the substrate and the absorber layer was investigated. The samples were tested under high-temperature exposure and under air and vacuum conditions measuring the solar absorptance and the emittance. Auger electron spectroscopy (AES) depth profiling studies were performed in order to investigate the changes of the element concentration caused by the ageing process. The behaviour of the diffusion barrier was examined in a two-layer system. A mathematical model for the grain boundary diffusion was developed and compared with the experimental results.