Compressive mica seals are being evaluated as an alternative seal for solid oxide fuel cells. This paper covers the comprehensive study of mica seals at the Pacific Northwest National Laboratory. Two types of micas are tested : Muscovite mica and Phlogopite mica. The major goal of mica seal development is to lower the leak rate and to increase the thermal cycle stability. In this paper we'll present two modifications of commercially available micas : hybrid micas, and infiltrated micas. High temperature leak rates will be reported as function of thermal cycles, mica thickness, compressive stresses, and sample size. In addition to the thermal cycling, mica seals were also subjected to mid-term (300-500 hours) stability tests in air and a wet reducing environments, and leak rate tested. Allowable leak rates based on thermaldynamic calculations will be presented and correlated with experimental results.