Thermal cycling was conducted on compressive mica seals at 800 degreesC in air. Thin (similar to0.1 mm) Muscovite mica was pressed between a metal pipe and an alumina substrate and tested for leak rates at a stress of 100 psi in the plain (mica only) and the hybrid design. The hybrid design involves adding two glass interlayers and was found to greatly reduce the leak rates in an earlier paper Two metals (Inconel #600 and SS430) with high and low coefficients of thermal expansion (CTE) were used to evaluate the effect of CTE mismatch on thermal cycling. The results showed that the leak rates were lower for the hybrid design than the plain micas. In addition, wing the lower CTE (SS430) metal pipe resulted in lower leak rates as compared to Inconel #600 metal (high CTE). In general, the leak rates increased with the number of thermal cycles: however, it tended to level off after several tens of thermal cycles. Microstructure examination using, canning electron microscopy revealed steps, indents, fragmentation and particle formation on the mica after thermal cycling. CTE measurement of the heat-treated Muscovite mica showed a relatively low value of similar to7 ppm/degreesC. The cause for the degradation of the mica is discussed.