Knudsen effusion vapor pressure measurements have been performed for vapor in equilibrium with Sc2O3(s). Experiments supported the conclusion that effusion was congruent. From experiments by Ames et al. (J. Phys. Chem. 1967, 71, 2707-2718) the vapor phase consisted of ScO(g), mol fraction 0.015 Sc(g), and O(g). Using the measured vapor pressures, values of the equilibrium constant for the vaporization reaction were calculated. From the equilibrium constant values and auxiliary thermodynamic data, the dissociation energy D-m(o)(T greater than or equal to 0) was calculated by the third-law method to be (670.81 +/- 8.87) kJ . mol(-1)