Nanocrystalline thoria was synthesized by temperature programmed decomposition of Th(CO₃₎₂ in an evolved gas analysis mass spectrometer set-up. The structural and stoichiometric changes encountered in the decomposition pathway were studied by off-line thermogravimetry (TGA), X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Accurateconversion temperature for transformation of Th(CO₃)₂→ThO₂ was arrived from the XPS mea-surements. Fouriertransforminfrared (FT-IR)measurementswere usedtocomparevibrational activities ofnanoand bulk polycrystalline thoria. Raman spectroscopic studies indicated optical phonon confinement effects in nanocrys-talline thoria. High resolution transmission electron microscopic examination on starting material, intermediates and nanocrystalline final product were carried out for studying the microstructure in the nanometer scale.