Two isomeric oxydianiline-based polypyromellitimides, P-44PM and P-34PM, were readily synthesized by the thermal solid-state polycondensation of the corresponding salt monomers, 44PME and 34PME, composed of 4,4’-oxydianiline and 3,4’-oxydianiline, respectively, and 2,5-di(ethoxycarbonyl)terephthalic acid derived from pyromellitic dianhydride. The thermal polycondensations of the salt monomers were conducted under reduced pressure or under high pressure, yielding highly crystalline polyimide oligomers. The P-44PM and P-34PM oligomers were subjected to solid-state post-polymerization at a high temperature up to 400 degrees C, leading to the formation of higher molecular weight polyimides. The P-44PM polymer postpolymerized at 400 degrees C had a high inherent viscosity of 1.11 dl g(-1). During the thermal post-polymerization, the crystal structure of P-44PM markedly changed.