A novel compressive Phlogopite mica seal was developed that showed superior thermal cycle stability with very low leak rates at 800degreesC. A commercially available Phlogopite mica paper was infiltrated with a wetting or melt-forming agent, and tested in a hybrid form during thermal cycling. The results of H3BO3-infiltrated mica showed a continued decrease in leak rates over thermal cycling, and very low rates (<5 x 10(-4) sccm/cm) were obtained after similar to15 thermal cycles. The results of Bi-nitrate infiltrated mica exhibited a constant leak rate of (1-4) x 10(-3) sccnVcm over 36 thermal cycles. The leak rates of the infiltrated mica were one to two orders of magnitude lower than leak rates for the as-received micas. Open circuit voltage tests were also conducted using dense 8YSZ plates to assess the effectiveness of the mica seals. Open circuit voltages equivalent to, or close to, the theoretical (Nernst) values were obtained. (C) 2004 Published by Elsevier B.V.