Platinic acid, H2Pt(OH)(6) hydrogen hexahydroxyplatinate (IV), was burned with benzoic acid and thermally decomposed to metallic platinum, liquid water, and oxygen gas in a micro-combustion calorimeter. The standard enthalpy of formation of platinic acid in the crystalline state was determined to be -(1287.9 +/- 3.6) kJ . mol(-1) at T = 298.15 K from the measured combustion energy. The thermal decomposition of platinic acid was examined by &s.c. and thermogravitnetric measurements.