A small continuous tubular reactor unit was constructed to study hydrothermal reactions. The unit was used in an experimental research program to study the kinetics of pressure precipitation of zirconium dioxide from aqueous solutions of zirconium tetrachloride. Experimental parameters investigated in the precipitation reaction included the temperature, the reactor residence time, and the feed composition. The significant findings of the study are related to the effect of reaction temperature on the overall rate of precipitation and the effect of feed composition on the precipitation procss, which under certain reaction conditions was found to involve an initially slow induction period followed by rapid precipitation. Based on the experimental results, a preliminary design concept was developed for a production-scale unit for manufacturing zirconia having a narrow particle-size distribution.