Composition control of copolyester was examined by using hydrodynamically induced phase separation, during solution polymerization, of p-acetoxybenzoic acid and m-acetoxybenzoic acid (m-ABA). Polymerizations were carried out in liquid paraffin at 1.0% of concentration under shear flow. The polymers were obtained as precipitates formed via phase separation of oligomers and the subsequent polymerization in the precipitates. When the feed ratio of m-ABA (chi(f)) was 20 mol %, the molar ratio of m-oxybenzoyl moiety in the precipitated polymers (chi(p)) was 5.3, 3.1, and 2.3 mol % at a shear rate (gamma) of 0, 147, and 489 s(-1), respectively. The values of chi(p) were much lower than those of chi(f), and shear flow lowered the value of chi(p); thus, controlling shear flow influenced both the reaction rate of the monomers and the miscibility of the oligomers. It especially enhanced the difference on the miscibility between homo-oligomers and co-oligomers. This change in the miscibility by shear flow brought about the more rapid precipitation of homo-oligomers than co-oligomers, leading to the selective formation of poly(p-oxybenzoyl).