Ultrahigh vacuum sealing technology has been developed for joining aluminum (Al) and stainless steel (SS) CF flange pairs, which provides a leak rate of less than 2 X 10(-11) Torr l/s following a bakeout at 150-degrees-C and return to room temperature. Sealing was achieved, in spite of the fact that the coefficient of linear thermal expansion of Al is about 1.5 times that of SS. It is assumed that the knife edges on the CF flanges changed diameters during bakeout without sliding of the knife edges with respect to the seal. This requires that the gasket assume the shape of a truncated cone. Sealing is retained during this process. This assumption was verified by three methods : (1) observing the transformation of the gasket from flat at 20-degrees-C to conical during bakeout at 150-degrees-C; (2) measuring the leak rate of CF 114, 203, 253, and 305 sealed flange pairs over the range from 20-degrees-C to 150-degrees-C, and; (3) inspecting the imprint of the knife edge of a CF 305 in the gasket after bakeout by profilometer trace and photographic examination of a section. The results showed that during bakeout, the shape of the gasket changed, accommodating the differential motion, and thus preventing damage to the seal at the knife edge/gasket interface.