A wide application of yttria particles for advanced ceramics has attracted a number of studies concerning the preparation of colloidal yttria particles. In the present work, near-monodisperse yttria particles are synthesized in urea aqueous solution by a homogeneous precipitation method. The effect of four experimental variables, such as concentrations of yttrium and urea, reaction temperature, and solution pH were characterized in terms of morphology and reaction kinetics. It was found that yttrium concentration, varying between 0.005 and 0.04 M, has a profound impact on the average size of particles, which systematically increases from 65 nm to over 220 nm. We also found that as yttrium concentration increases, not only does the size distribution broaden, but particles also start to agglomerate above 0.025 M of concentration, mainly due to the reduction of zeta potential. The rate of precipitation reaction, however, is shown to be independent of yttrium concentration. In contrast, as urea concentration increases from 0.04 to 4.0 M, the average particle size exhibits a gradual decrease from ca. 220 to ca. 100 nm. At extremely high urea concentration such as 7.0 M, a significant level of inter-particle agglomeration is observed. The rate of precipitation is found to increase with urea concentration up to 3.0 M. Above 3.0 M, the concentration dependence is weakened. Temperature mainly affects the kinetics of precipitation, not thermodynamic quantities such as average particle size, which is a strong function of the concentrations of reactants (both yttrium and urea). Assuming Arrhenius-type reaction kinetics, the activation energy for precipitation is obtained as 29 kcal/mol. This value is quite comparable with the activation energy of urea decomposition (28-32 kcal/mol). Based on this, we propose that urea decomposition may be a rate-determining step in the formation of yttria particles by homogeneous precipitation method. Particle morphology and the reaction kinetics are also sensitive to solution pH. While at low pH (<2.0) particles experience severe agglomeration, and the rate of precipitation is slow. As pH increases above 3.0, near-monodisperse yttria particles are obtained. Lastly, based on two observations that, first, pH does not change during precipitation (maintaining pH 5.5-6.0), and second, the equilibrium constant of yttrium ion hydrolysis at this pH (i.e., K=[Y(OH)(2) +]/[Y3+]) is too small to serve as a reasonable source of hydroxyl ions in the final yttria compound, we propose, along with some experimental supports, a new reaction mechanism that is more plausible in yttria particle precipitation from urea aqueous solution. (C) 2004 Elsevier B.V. All rights reserved.