Three series of isomeric poly(amide imide)s (series III, IV, and V) were synthesized by the direct polycondensation of 2,2'-bis(4-aminophenoxy)biphenyl (2,2'BAPB), 4,4'-bis(4-aminophenoxy)biphenyl (4,4'-BAPB), or their equimolar mixture (2,2'-BAPB/4,4'-BAPB = 1/1) with 12 diimide diacids and with triphenyl phosphite and pyridine as condensing agents. A comparison of the physical properties of these three series was also made. The inherent viscosities of series III, IV, and V were 0.25-0.84, 0.25-1.52, and 0.43-1.30 dL g(-1), respectively. Most of the series III polymers showed better solubility because of the non-para structure, with the solubility order found to be III > V > IV. According to X-ray diffraction patterns, the amorphous poly(amide imide)s had excellent solubility, whereas the crystalline polymers were less soluble. All the soluble polymers afforded transparent, flexible, and tough films, which had tensile strengths of 57-104 MPa, elongations at break of 3-20%, and initial moduli of 2.05-2.86 GPa. The glass-transition temperatures (measured by differential scanning calorimetry) were highest for series IV, which contained the rigid 4,4'-biphenyl units (254-299 degreesC); copolymer series V ranked second (237-277 degreesC), and series III, with crank 2,2'-biphenyl structures, had the lowest values (227-268 degreesC). The 10% weight-loss temperatures (measured by thermogravimetric analysis) were close to one another, ranging from 527 to 574 degreesC in nitrogen and from 472 to 543 degreesC in air.