Thermal decomposition of cerium(III) acetate hydrate, Ce(CH₃CO₂)₃·1.5H₂O, to cerium(IV) oxide, CeO₂, in helium has been successfully investigated by sample-controlled thermogravimetry combined with evolved gas analysis by mass-spectrometry (SCTG–MS). Cerium(III) anhydrous acetate decomposed to cerium (IV) oxide through four decomposition steps in the temperature range of 250–800 °C. SCTG–MS was very useful to distinguish the successive decomposition accompanying the formation of the intermediate products to identify simultaneous gas evolution during the mass losses. The decomposition intermediates quenched from SCTG were characterized by X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES). The XANES revealed clearly the coexistence of Ce(III) and Ce(IV) and the valence change from cerium(III) to cerium(IV). The three decomposition intermediate products were presumed to be Ce₂O(CH₃CO₂)₄, Ce₂O₂(CH₃CO₂)₂ and Ce₂O₂CO₃. A detailed thermal decomposition mechanism of Ce(CH₃CO₂)₃·1.5H₂O is discussed.