Effect of acid acceptor (AA) on the low temperature solution polymerization of the copolyamide 4I-PIP(20) from isophthaloyl chloride, 4,4＇-diaminodiphenylsulfone, and piperazine (their diamine component ratio is 80-20) was studied. The type and amount of AA had a large effect on not only the polymerization condition and polymerization degree of 4I-PIP(20), but also on its microstructure. Pyridine was the most favorable AA, since it gave a homogeneity to the polymerization solution of 4I-PIP(20), and that it was most suitable for the synthesis of 4I-PIP(20) with high polymerization degree. The H-NMR measurement made it clear that 4I-PIP(20) prepared with pyridine was random copolyamide. (C) 2000 John Wiley & Sons, Inc.