Abstract: The chemical equilibriums of zinc sulfate heptahydrate at 25~1 100 ℃ have been calculated by the method of Gibbs free energy minimization，meanwhile the thermal decomposition of the hydrates was investigated by TG-DTA.There were two stages approximately in the thermal decomposition of zinc sulfate heptahydrate：one was the dehydration of the hydrates，and another one was the thermolysis of the anhydrous salt.The dehydration occured at range of room temperature to 311 ℃，while the thermolysis at 687~1 050 ℃.The solid phase dehydration products contained ZnSO4·6H2O，ZnSO4·H2O， ZnSO4·xH2O［x∈（0.8，0.9）］，and so on.The solid thermolysis products contained the intermediates ZnO2·ZnSO4 and the final solid phase product was ZnO.The results showed that there was internal consistency between Gibbs free energy minimization and TG-DTA. 

Key words: Gibbs energy minimization, thermal decomposition, thermal analysis, zinc sulfate heptahydrate