Several kinds of metastable compounds, pseudo zirconium oxide sulphates (PZOS) with a chemical composition of Zr3O5SO4 . nH(2)O, were previously synthesized by the thermal hydrolysis of solutions containing zirconium sulphate at 200 or 240 degrees C. The obtained PZOS samples were again hydrothermally treated in different sulphuric acid solutions (<1.0 mol l(-1)) at 240 degrees C, and their hydrothermal decomposition behaviour was investigated by TEM observation. The PZOS samples mostly crystallized to plate-like zirconium oxide sulphate (ZOS) in the concentrated sulphuric acid solution (>0.5 mol l(-1)), but long-whiskered monoclinic ZrO2 crystals grew with decomposition of the PZOS samples obtained from the starting mixtures with added Zr(OH)(4) when rapidly heated to the hydrothermal treatment temperatures. It was found that many ultrafine monoclinic ZrO2 crystals were simultaneously formed during the hydrothermal preparation of the PZOS samples, and during the following hydrothermal decomposition of the PZOS samples, the whiskered crystals of monoclinic ZrO2 grew with the consumption of PZOS from the coexisting ultrafine monoclinic ZrO2 particles which act as seed crystals.