The thermal decomposition of zinc acetate dihydrate Zn(CH3CO2)(2).2H(2)O in some humidity-controlled atmospheres has been successfully investigated by novel thermal analyses, which are sample-controlled thermogravimetry (SCTG), thermogravimety combined with evolved gas analysis using mass spectrometry (TG-MS) and simultaneous measurement of differential scanning calorimetry and X-ray diffractometry (XRD-DSC). The thermal processes of anhydrous zinc acetate in dry gas atmosphere by conventional linear heating experiment initiated with the sublimation around 180 degreesC, followed by the fusion and the decomposition over 250degreesC. SCTG was useful to interpret clearly the successive reaction because the high-temperature parallel decompositions were effectively inhibited. The thermal behavior changed dramatically by introducing water vapor in the atmosphere and the thermal process was quite different from that in dry gas atmosphere. Zinc oxide (ZnO) was formed only in a humidity-controlled atmosphere, and could be easily synthesized at temperatures below 300degreesC. XRD-DSC equipped with a humidity generator revealed directly the crystalline change from Zn(CH3CO2)(2) to ZnO. A detailed thermal process of Zn(CH3CO2)(2)-2H(2)O and the effect of water vapor are discussed.