A diimide dicarboxylic acid, 1,4-bis(4-trimellitimidophenoxy)naphthalene (1,4-BTMPN), was prepared by condensation of 1,4-bis(4-aminophenoxy)naphthalene and trimellitic anhydride at a 1 : 2 molar ratio. A series of novel poly(amide-imide)s (IIa-k) with inherent viscosities of 0.72 to 1.59 dL/g were prepared by triphenyl phosphite-activated polycondensation from the diimide-diacid 1,4-BTMPN with various aromatic diamines (Ia-k) in a medium consisting of N-methyl-2-pyrrolidinone (NMP), pyridine, and calcium chloride. The poly(amide-imide)s showed good solubility in NMP, N,N-dimethylacetamide, and N,N-dimethylformamide. The thermal properties of the obtained poly(amide-imide)s were examined with differential scanning calorimetry and thermogravimetry analysis. The synthesized poly(amide-imide)s possessed glass-transition temperatures in the range of 215 to 263 degrees C. The poly(amide-imide)s exhibited excellent thermal stabilities and had 10% weight losses at temperatures in the range of 538 to 569 degrees C under a nitrogen atmosphere. A comparative study of some corresponding poly(amide-imide)s also is presented.