The thermal decomposition behavior of metal-oxygen clusters with Dawson structure, YmHnP2M18O62#8226;qH2O, was systematically investigated using the X-ray power thermodiffractometry and FTIR at different temperatures, TG-DTA. The experimental results indicated that Dawson-type metal-oxygen clusters with anion,[P2Mo18O62]6-,and different counter cations,NH4+,Cu2+,K+,underwent an irreversible thermal rearrangement around 210 ℃,240 ℃,250 ℃, respectively to give the corresponding Keggin-type metal-oxygen clusters containing [PMo12O40]3-as one of the products. For the metal-oxygen clusters containing tungsten, the pyrolysis procedure of potassium salt with [P2W18O62]6-anion is similar to that of metal-oxygen clusters with [P2Mo18O62]6-anion, whenas the NH4+, Cu2+salts of [P2W18O62]6-seem to decompose directly into WO3 at 400 ℃, 450 ℃,respectively.These results testify that the thermal solid-state rearrangements of Dawson-type anions correlated with the thermal stability of the corresponding Keggin-type compounds. Furthermore, some proposals about the criterion and the adjudication of thermal decomposition temperature for metal-oxygen clusters with different thermal decomposition behavior are also given.