Aromatic processable poly(amide-imide)s have been prepared according to the Yamazaki and Higashi phosphorylation reaction from flexible aromatic diamines and a mixture of isomeric 1,2,4-benzenetricarboxylic acid monoethylesters following a novel "one-pot" procedure. The polymerization was performed in 1-methyl-2-pyrrolidone/LiCl solutions using triphenylphosphite and pyridine as condensing agents to form amide bonds. Cyclization of the intermediate poly(amide-amic ethylester) occurred by eliminating ethanol under the polymerization conditions used, thus leading to the formation of imide rings. The influence of several parameters which regulate the degree of polymerization and the formation of the imide rings, such as reaction temperature, triphenylphosphite/monomers molar ratio, and pyridine concentration was carefully investigated. In particular, the catalytic activity of pyridine in the imidization reaction has been demonstrated. C-13-NMR spectroscopy has been used to show the absence of constitutional regularity in the prepared PAIs thus indicating equal reactivity toward polycondensation of the two different carboxyl groups of the 1,2,4-benzenetricarboxylic acid monoethylesters.