The ΔH of formation of I2– and (CNS)2– in aqueous solution have been obtained from measurements of the equilibrium constants of the reactions I + I–→ I2– and CNS + CNS–→(CNS)2–. They are 5·6 and 6·4 kcal. (g. ion)–1 respectively. From the former ΔHf°(I2–)aq and ΔGf°(I2–)aq have been calculated to be + 2·5 and + 1·0 kcal. respectively. The electron affinity of I2 in solution and in the gas phase are estimated to be 4·6 and 2·4 ± 0·5 eV.