The polymerization of poly(p-oxybenzoyl) (POB) from p-acetoxybenzoic acid was examined in the presence of three kinds of aromatic monofunctional compounds such as p-hexyloxybenzoic acid (HOBA), p-decyloxybenzoic acid (DOBA) and p-octadecyloxybenzoic acid (OOBA) under unstoichiometric polymerization conditions. Polymerizations were carried out in liquid paraffin at 320 degreesC. POB crystals were obtained by reaction-induced crystallization of oligomers. The obtained POB crystals were found to be consisted of high molecular weight polymers compared with the products obtained by the melt polymerization. Polycondensation proceeded under unstoichiometric condition through following mechanisms; When DPn of oligomers exceeds a critical value, they are crystallized to form the crystals. End-free oligomers are preferentially crystallized due to the lower solubility over end-capped oligomers. End-capped oligomers are also crystallized, and polycondensation proceeds with eliminating end-capping groups of oligomers by trans-esterification just when they are crystallized (mechanism 1). Though a part of the end-capped oligomers are contained in the crystals, end-capping groups are excluded by solid-state polycondensation (mechanism 2). The mechanism 1 is major path in the system of DOBA and OOBA due to the bulkiness of oligomer end groups, and both mechanisms 1, and 2 occur in the system of HOBA.