Preparation of poly(p-oxycinnamoyl) (POC) microspheres was examined by the reaction-induced phase separation of oligomers during the polycondensation of (E)-4-acetoxycinnamic acid in various solvents. Polymerization in liquid paraffin yielded the microspheres having a smooth surface at 270330 degreesC, of which the average diameter was 4.6 mum. In contrast to this, the morphology of the products was drastically changed in Therm S 800 (TS8) and Therm S 900 (TS9) by polymerization temperature. When the polymerization was carried out at 310 degreesC in TS8 and 300 degreesC in TS9, the microspheres were obtained, of which the average diameter was 3.9 and 3.3 mum, respectively. The platelike crystals were formed under these temperatures, of which thickness was approximately 0.1 mum. The formation of the microspheres was highly related to the miscibility between the oligomers and the solvents. The average diameter of the microspheres increased with the decrease of the miscibility, and this tendency could be accounted for by the interfacial tension between two immiscible liquid phases. The microspheres were formed through the formation of the microdroplets by the liquid-liquid phase separation of the oligomers and the subsequent solidification of the microdroplets due to the further polymerization in them.