Aliphatic polyimides having inherent viscosities of 0.4-2.8 dL g-1 were readily synthesized by the melt or solid-state polycondensation under high pressure of the salt monomers composed of aliphatic diamines having 6-12 methylene chains and 4,4’-oxydiphthalic acid at 140-330-degrees-C under 250-650 MPa. It is notable that the high-pressure polycondensation proceeded rapidly with the elimination of water in the closed reaction vessel and was almost complete within 3 h. The polyimides having six and eight methylene units were highly crystalline yellowish white solids, whereas those with odd number carbons were amorphous pale yellow resinous products. The thermal behavior of the polyimides are also discussed.