Solvent effect on the morphology of poly(p-oxybenzoyl) (POB) prepared by the reaction-induced phase separation of oligomers was examined by the polymerization of p-acetoxybenzoic acid in perfluoropolyether Aflunox(TM) (AFL2507 and AFL606). Polymerization was carried out at 320degreesC for 6 hr. POB microspheres were formed in AFL2507 by the liquid-liquid phase separation of oligomers due to the low miscibility of oligomers in AFL2507. The molecular weight of the solvent influenced the morphology, and the polymerization in AFL606 of which the molecular weight was lower than AFL2507 yielded whiskers formed by crystallization of oligomers induced by the increase in miscibility compared with that in AFL2507. The solvent structure and its molecular weight influenced the miscibility of oligomers and ultimately controlled the morphology from whisker to microsphere. Copyright (C) 2004 John Wiley Sons, Ltd.