Abstract: We have previously reported the solvothermal synthesis of pure monoclinic and tetra nal zirconia nanoparticles in water and methanol. The solvent effect on this synthesis was studied further for this work. The effects of reaction temperature, reaction time and thermal treatment temperature on the crystal phases of the as-prepared hydrous ZrO2 precipitates formed from the solvothermal reaction of hydrated zirconyl nitrate (ZrO(NO3)2·2H2O) and urea were characterized by X-ray diffraction. Plausible mechanisms for the formation of hydrous precipitates, their transformation and their stabilization were examined. Precipitates formed initially with a tetra nal structure irrespective of the solvent used. These tetra nal precipitates, in water, transform into thermodynamically stable monoclinic structures as a result of higher precipitate solubility under hydrothermal conditions. Dissolution and precipitation processes are thus favored. The tetra nal structure was, however, maintained under methanothermal conditions probably due to poor precipitate solubility in methanol. The removal of water from precipitates by reaction with urea in methanol may make the tetra nal structure more rigid. This contributes to the maintenance of the tetra nal structure after precipitate thermal treatment (at 400 ℃). Differences in precipitate solubility and their reaction with urea in water and methanol may dictate the synthetic pathway for pure monoclinic and tetra nal ZrO2 via the solvothermal method.