A novel method of thermogravimetry under controlled mercury and oxygen partial pressures was developed, which allowed us to obtain the precise thermodynamical data for the equilibrium M HgO 2 (s) ⇔ M O(s) + Hg(v)+1/2O 2 ( M =Ca, Sr, and Ba). Temperature dependencies of equilibrium constants K p are described by the equations: log K p (CaHgO 2 ) =− (9424±540)/ T + (10.28±0.57), (888–1013 K); log K p (SrHgO 2 )=−(10197 ±890)/ T + (10.10±0.83), (1043–1118 K); log K p (BaHgO 2 ) = − (10474±730)/ T +(9.94±0.64), (1098–1163 K). Thermal stability of mercurates increases in the order CaHgO 2 , SrHgO 2 , BaHgO 2