Solid products were prepared from mixtures of zirconyl carbonate and sulphuric acid solution under thermal hydrolysis at 240 degrees C. These crystalline products and their formation mechanism were investigated by evaluating the amount of products, and by X-ray diffraction analysis and TEM observation. The solid products precipitated were zirconium oxide sulphates (ZOS, PZOS, ZrOSO4 . H2O) and m-ZrO2. The area and thickness of the fine plate-like crystals of ZOS increased with increasing concentration of the solution and hydrothermal treatment time. For thin starting solutions, the precipitation velocity and the amount of PZOS increased. For thick starting solutions, those of ZOS increased. For 0.5 mol/l(-1) ZrOSO4 solution (ZS0.5) from the thin solution, PZOS crystallized early and m-ZrO2 crystallized by releasing sulphuric acid ions in PZOS with treatment time. For ZS2.5 from the thick solution, ZrOSO4 . H2O was crystallized by increasing the proportion of sulphuric acid ions to zirconium ions in the solution, which was caused by ZOS precipitation.