The crystal of p-aminohippuric acid was polymerized in the solid state by annealing at a temperature below its melting point. The reaction was carried out on the p-aminohippuric acid crystal at 170°C for 260 h under reduced pressure. The specimens formed through the thermal solid state polycondensation procedure from original monomer single crystals were examined mainly by means of differential thermal analysis (DTA), X-ray diffractometry and scanning electron microscopic (SEM) observation. In DTA measurements, whereas the endothermic spike peak of a monomer single crystal was observed approximately at 200 °C, the endothermic peak of a polymerized specimen appeared approximately at 380°C