Bourdon gauge measurements of samples of I2(g) plus (s) have been made in the temperature range 900°–1200°K and over a fourfold range in pressure. The pressure vs. temperature behavior of the system can be described satisfactorily by the equilibria, The standard enthalpy and entropy of the first reaction between 900°–1200 °K were calculated using the second law with the result; , . The standard heat of formation, and entropy, were found to be 44.2 kcal and 15.4 e.u., respectively.