Well characterised. polycrystalline powders of commercially procured CaCO3 were thermally decomposed in the vacuum as well as in the flowing gas atmosphere for the purpose of studying solid state transformations. The characterisation of tbe end product CaO, obtained from the thermal decomposition, revealed contrasting features in the powder X-ray diffractograms. While the flowing gas method, conducted inside a thermogravimetric analyser (TGA), indicated formation of stable microcrystalline calcia, the decomposition under dynamic vacuum revealed formation of metastable-nanocrystalline calcia. The latter study was carried out in an evolved gas analysis-mass spectrometry (EGA-MS) facility. Experiments were also conducted inside the high temperature XRD (HTXRD) machine. The paper attempts to bring out possible mechanisms responsible for formation of these end products with such glaring structural contrast. Non-equilibrium conditions prevalent under dynamic vacuum condition as well as misfit strain energy available from CaCO3/CaO interface are presumed to be the reason behind such metastable transformations. Kinetic analysis of the transformation revealed prevalence of nucleation and growth phenomena. Corresponding Arrhenius factors were also calculated.