PVRC project 96-09G is the first of a series of pressurized media tests and it investigates the effect of long term pressurized hot air exposure on the performance of two types of reinforced flexible graphite based gaskets, gasket A with a thin flat metal reinforcement, and gasket B with a thick corrugated metal. The gaskets are tested in NPS 3, Class 150 bolted flanged joints at 700°F under a constant internal air pressure of 110 psi. Gaskes were aged until about 90% relaxation of their initial compressive stress was achieved. Very different rates of stress relaxation and leak rate increase over time are observed for the two gasket types. In the case of gasket A, the degradation of the properties was relatively slow and the test lasted 342 days. For gasket B, a much faster rate of degradation was recorded during two tests that lasted respectively 34 and 46 days. This behavior was not expected and cannot be explained for the time being.A comparison made between predicted stress relaxation and experimentally measured long term stress relaxations of gasket A demonstrates that the PVRC quantification tools Ag and Qpg developed for RFG sheet gasket qualification, are making accurate service life predictions in the case, of flexible graphite sheet gaskets with a thin flat metal reinforcement.