Composition control of aromatic poly(thioester-amide) was examined by the reaction-induced phase separation during polymerization of S-acetyl-4-mercaptobenzoic acid (AMBA) and p-acetylaminobenzoic acid (AABA) in aromatic solvent. The poly(thioester-amide)s were obtained as precipitates and their yields became lower at the middle range of the content of AMBA in feed ((f)). The contents of p-mercaptobenzoyl (MB) moiety ((p)) in the precipitates prepared without shearing were in good agreement with the (f) values. In contrast to this, the (p) values of the precipitates prepared at (f) of 50-70 mol % under shearing were much lower than the (f) values. The reaction rate of AMBA increased with shearing, whereas that of AABA was unchanged by shearing. This shearing effect on the reaction rates accelerated to form the homo-oligomers. The solubility of MB oligomers enhanced by shearing, whereas that of p-benzamide oligomers did not enhance owing to the strong interaction through hydrogen bonding. The MB oligomers were inhibited to be precipitated, resulting in the lower (p) values than the (f) values. The composition could be controlled by the application of the shearing to the heterogeneous polymerization. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4301-4308