The polymorphic transition and the thermal decomposition of vaterite are characterized. The effect of the morphology of the parent vaterite upon the polymorphic transition as well as upon the decomposition of pseudomorphous ex-vaterite, i.e. "spherulitic" calcite, is presented. It is shown for the first time that vaterite can be decomposed into calcium oxide and carbon dioxide before being transformed into calcite. The nature of the evaluated kinetic parameters and derived mechanistic interpretations are discussed.