Factors that affect the size, size distribution, and morphology of alpha-Fe2O3 nanoparticles obtained via continuous hydrothermal synthesis have been investigated in this work. The presence of polyvinyl alcohol (PVA) during synthesis is shown to limit the aggregation of particles and to produce narrow particle size distributions than in the case when PVA is absent. Narrow particle size distributions are also obtained with an increase in PVA concentration, beyond a minimum concentration required to cover all particles with a polymer layer. The average particle size is shown to increase with temperature and residence time, and is accompanied by morphology changes in some cases. Both modeling and experimental studies indicate that equilibrium species distributions do not affect the particle morphology, so that changes in morphology in this work are probably due to kinetic phenomena. (c) 2007 Elsevier B.V. All rights reserved.