As optical coatings become more complex, partly due to improved deposition processes and more stringent design criteria, the number of layers in the coating increases, with a corresponding risk of interfacial instability. This can lead to partial film delamination, arising from residual stress effects, from interdiffusion of different chemical species across the film boundaries, or from the presence of impurity species at the interface itself. These effects are very relevant to the application of such coatings in laser systems, especially where the newer digital techniques are used to synthesise graded designs. One useful way of exploring the effect is to incorporate a large number of interfaces within a Fabry-Perot etalon cavity, and to temperature cycle the structure, whilst measuring its spectral response. The changes observed can be correlated with information derived from X-ray photoelectron spectroscopic examination of discrete interfaces specifically fabricated to simulate those present in the etalon cavity.