This paper presents sealing and mechanical properties of isotropically expanded PTFE gasket sheets, characterized by the room temperature tightness test (ROTT), developed by the Pressure Vessel Research Council (PVRC). The expanded PTFE sheets are highly fibrillated and porous. Three types of sheets were tested, including two regular 1/16 and 1/8-in. thick sheets of density 0.85 g cm(-3), and one 1/8-in. thick rigid type gasket sheet of density 1.2 g cm(-3). Helium mass leak rates and gasket deflection values were measured. The regular 1/16 in. sheet leaks less than the 1/8 in, sheet by 2-4 times, and the regular and rigid 1/8 in. sheets show comparable leakage. All the tests reveal a drastic improvement in sealability between gasket stress of 1025 and 5390 psi; above that, the leakage still steadily decreases, although improvement occurs at a lower rate. The deflection results, when adjusted with initial thickness, are similar between the regular 1/16 and 1/8 in. sheets, but lower in value for the rigid sheet due to its higher density, and hence lower compressibility. Both leakage and gasket deflection results indicate very similar sealing and mechanical behaviors for all types of expanded PTFE products. The results of expanded PTFE sheets are further compared with those of a skived, virgin PTFE sheet and a filled PTFE sheet. The deflection results confirm the high compressibility, low recovery and excellent creep resistance for the expanded PTFE. These property differences are attributed to the differences in micro-structures of the materials. The comparison shows that the expanded PTFE sheet seals the tightest and creeps the least up to 6500 psis. Tightness hardening effect has greatly limited the sealing performance of the skived and filled PTFE at such high gasket stresses. In addition, the sealing and mechanical behaviors of virgin and filled PTFE are less consistent than expanded PTFE in terms of test-to-test and thickness variations. Gasket selection criteria based on the ROTT test data are discussed. Further, the tightness concept, tightness data, and PVRC gasket constants associated with the test results are presented.