Polymerization of 4-acetoxybenzoic acid (ABA) with 3,5-diacetoxybenzoic acid (DABA) was examined to create a novel morphology of poly(p-oxybenzoyl) (POB) by means of the phase separation of oligomers during polymerization. Polymerizations were carried out at a concentration of 1.0% in liquid paraffin at 320 degreesC. Polymerization of ABA yielded the POB whiskers. On the other hand, the polymerization of ABA with DABA of which the concentration in the feed (chi(f)) was 0.05-0.20 yielded the microspheres having needlelike crystals on the surface. The average diameter of the microspheres was in the range of 3.4-1.6 mum and the average length of the needlelike crystals was 3.2-0.3 mum. The diameter and length decreased with chi(f). DABA acted as a liquid-liquid phase separation inducer and the liquid-liquid phase separation of co-oligomers comprising 4-oxybenzoyl units and 3,5-dioxybenzoyl units was induced in the beginning of polymerization to form the core microspheres. Then the phase separation mode was changed to the crystallization of the homooligomers of the 4-oxybenzoyl unit and the homooligomers were crystallized as needlelike crystals on the surface of microspheres already precipitated. Solid-state polymerization occurred in the precipitates. The microspheres having needlelike crystals were prepared by the combination of liquid-liquid phase separation and the crystallization of oligomers during solution polymerization. The obtained microspheres having needlelike crystals possessed very high crystallinity and exhibited good thermal stability.