The enthalpy of reaction of metallic calcium with perchloric acid was measured for the first time in a sealed swinging calorimeter equipped with an isothermal shell. Standard enthalpies of formation of cal cium ion in an infinitely diluted aqueous solution (-542.8 ± 1.0 kJ/mol) and calcium chloride in crystal state (-794.9 ± 1.0 kJ/mol) were calculated according to the results obtained with the use of published data.