Thermal decomposition of Zr(C₂O₄)₂·4H₂O within an autoclave or in a conventional tube furnace at temperatures below 380 °C resulted in nano- and micron-sized ZrO₂, respectively. Reactions under autogenic pressure yielded monodisperse monoclinic (m) and tetragonal (t) ZrO₂ nanoparticles with an average diameter of 8 nm and interconnected t-ZrO₂ nanoparticles with diameters of 4 nm, depending on the synthesis temperature. Samples were characterised by X-ray diffraction (XRD), small angle X-ray scattering (SAXS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) associated with energy dispersive X-ray spectroscopy (EDS), Raman microspectroscopy and photoluminescence spectroscopy (PL). Nanostructured zirconia materials exhibited high specific areas of 276–385 m² g⁻¹ which make them promising candidates as catalysts and catalyst supports. Co-existence of m- and t-ZrO₂ nanoparticles with diameters of 6–9 nm, i.e. above the critical particle size of 6 nm for the formation of t-ZrO₂, demonstrated that the particle size is not the only factor for stabilisation of the t-ZrO₂ modification at room temperature.