Commercially available Phlogopite mica papers of varying thickness, similar to0.1, similar to0.2, and similar to0.5 mm, were evaluated as potential solid oxide fuel cell (SPFC) seal materials. The micas were tested in two forms: plain and hybrid. The hybrid form involved the addition of glass interlayers between the mica and the adjacent components. For each sample, about 30 thermal cycles were conducted and the 800degreesC leak rates were determined. The results showed an excellent thermal cycle stability of the Phlogopite micas in the hybrid design in that the leak rates remained almost constant after similar to10 cycles. In addition, the leak rate appeared to increase with increasing mica thickness in the hybrid design, but showed no thickness dependence for mica in the plain design. The Phlogopite micas also showed good mid-term (similar to500h) stability in both air and reducing environments. Microstructure characterization showed no distinct degradation such as fragmentation and particle formation after thermal cycle and the mid-term stability tests. (C) 2003 Elsevier B.V. All rights reserved.