The effects of reaction parameters on the synthesis of urea-terminated poly(1,6-hexamethyleneurea) particles dispersed in a polyol continuous phase have been studied. Oligomeric units are held together by hydrogen bonding into a macrostructure which separates as a stable dispersed phase during synthesis. Monomer addition rate, synthesis temperature and stirring rate are important factors in morphology control of the particles. High aspect ratio (10-20) particles having a spiral fibre bundle morphology (by scanning electron microscopy) are produced at synthesis temperature between 140 and 175 degrees C; spherical particles are produced at 185 degrees C. Particles produced at 140 degrees C have a different crystalline structure from particles produced at higher temperatures. Molecular weight, particle size and the extent of hydrogen bonding increase with increasing synthesis temperature. The structure of these highly crystalline particles is established at an early stage in the reaction, Annealing up to 185 degrees C has little effect on particle morphology. Annealing near the melting point (270 degrees C) results in loss of crystallinity; particle macrostructure is destroyed by melting or dissolution.