Soluble poly(amide imide) derivatives were prepared through the direct polycondensation of 1,2,4-benzenetricarboxylic acid and three diamines-bis[4-(3-aminophenoxy)phenyl]sulfone, bis(4-aminophenyl)-1,4-diisopropylbenzene, and 4,4'-oxydianilne-in the presence of metal salts and phosphorous compounds. Phosphonium salt, which was used as the initiating species and was prepared by the reaction of the metal salts and phosphorous compounds, reacted with 1,2,4-benzenetricarboxylic acid to form acyloxy phosphonium salt, and then the salt was reacted with a diamine for the preparation of the prepolymers. The prepolymers were converted into the corresponding poly(amide imide)s in a homogeneous solution state at 180degreesC. The poly(amide imide)s showed good thermal and mechanical properties. Glass-transition temperatures were observed from 240 to 270degreesC in differential scanning calorimetry traces. A melting endotherm was not observed for the polymers with differential scanning calorimetry. The initial decomposition occurred around 400degreesC according to thermogravimetric analysis, and major weight loss was observed from 610 to 680degreesC. The poly(amide imide)s had comparatively good solubility in aprotic polar solvents at concentrations high enough (similar to30%) for the fabrication of various forms.