A series of new aromatic poly(amide-imide)s (PAIs) was synthesized by triphenyl phosphite-activated polycondensation of the diimide-diacid, 1,4-bis(trimellitimido)-2,3,5, 6-tetramethylbenzene (I), with various aromatic diamines in a medium consisting of N-methyl-2-pyrrolidone (NMP), pyridine, and calcium chloride. The PAIs had inherent viscosities of 0.82-2.43 dL/g. The diimide-diacid monomer (1) was prepared from 2,3,5,6-tetramethyl-p-phenylenediamine with trimellitic anhydride (TMA). Most of the resulting polymers showed an amorphous nature and were readily soluble in a variety of organic solvents including NMP, N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF). Transparent, flexible, and tough films of these polymers could be cast from DMAc solutions. Their cast films had tensile strengths ranging from 80 to 95 MPa, elongation at break, from 10 to 45%, and initial modulus from 2.01 to 2.50 GPa. The 10% weight loss temperatures of these polymers were above 510 degrees C in nitrogen.