The vaporization of RbI(s or l) has been investigated over the temperature range 750 to 936 K by mass spectroscopy, mass-loss Knudsen effusion, and X-ray powder diffraction. The vapor phase in equilibrium with RbI(s or l) consists of RbI(g) and Rb 2I 2(g). Thermodynamic values were determined for the following: RbI(s) = RbI(g), reaction (1); 2RbI(s) = Rb 2I 2(g), reaction (2); 2RbI(g) = Rb 2I 2(g), reaction (3). From the slopes and intercepts, the following standard enthalpy and entropy changes with associated standard deviations were derived at the median temperature 843 K: for reaction (1), ΔH o = (43.4 0±0.4 4) kcal th mol −1 and ΔS o = (30.3 4±0.5 2) cal th K −1 mol −1; for reaction (3), ΔH o = (51.2 9±0.4 5) kcal th mol −1 and ΔS o = (34.6 0±0.5 4) cal th K −1 mol −1; for reaction (3), ΔH o = −(35.5 2±0.4 4) kcal th mol −1 and ΔS o = −(26.1 0±0.5 2) cal th K −1 mol −1. Based on the values at their median temperature the calculated values at 298.15 K are as follows: for reaction (1), ΔH o = (45.7 2±0.4 2) kcal th mol −1 and ΔS o = (34.6 3±0.5 0) cal th K −1 mol −1; for reaction (2), ΔH o = 54.8 4±0.4 2) kcal th mol −1 and ΔS o = (41.1 0±0.5 0) cal K −1 mol −1; for reaction (3), ΔH o = −(36.5 9±0.4 4) kcal th mol −1 and ΔS o = −(28.1 6±0.5 3) cal th K −1 mol −1. By use of the third-law procedure, the following ΔH o values were derived at 298.15 K: for (1), (46.8 5±0.1 5)kcal th mol −1; for (2), (56.8 2±0.1 9) kcal th mol −1; for (3), −(36.8 8±0.1 4) kcal th mol −1. The thermodynamic values are compared with previously determined values.