Submicron spherical particles of the yttria precursor Y(OH)CO[subscript]3·H[subscript]2O were continuously precipitated in three types of reactors: (1) mixed-suspension mixed-product removal (MSMPR), (2) semi-batch, and (3) packed bed. The homogeneous precipitation occurred as a result of the thermal decomposition of urea in an aqueous yttrium nitrate solution;A wide variety of particle size distributions were produced from these reactors. Mathematical models, which assumed that particle growth was by a diffusion mechanism, successfully allowed the size and shape of the particle size distributions to be predicted and controlled. It was shown that the concentration of the rate-limiting diffusing species could be calculated from a simple analysis of the particle size distribution from the MSMPR reactor. From such an analysis, it appeared that carbonate was the rate-limiting diffusant.