The polycondensation of isophthalic acid (IPA)terephthalic acid (TPA) with aromatic diols by tosyl chloride/dimethylformamide/pyridine in solution was examined through changes in the IPA/TPA compositions, the kinds of dihydroxyl components, the periods of their addition, and the reaction temperatures. The reaction proceeded favor-ably at IPA/TPA ratios of 70/30 to 50/50, similarly to an earlier report on the interfacial reaction. The effects of the compositions were significant in the reactions with monosubstituted hydroquinones. The results were examined from distributions of the resulting oligomers prepared at a reaction extent of 0.7, determined by gel permeation chromatography. The reaction producing better results exhibited distributions closer to the theoretical ones. The period of addition also favorably affected the distributions as well as the results of the polycondensation. These results were attributed to the change in the reaction method, in which the diols reacted with the aggregates that formed from the activated IPA and TPA. The change was likely caused by the degree of association of IPA and TPA in the aggregates, on the basis of melting points and IR spectra of mixtures of dimethyl esters of IPA and TPA prepared by the quenching of the aggre-gates with methanol. (C) 2004 Wiley Periodicals, Inc.