The thermodynamic stability of rubidium thorate, Rb₂ThO₃(s), was determined from vaporization studies using the Knudsen effusion forward collection technique. Rb₂ThO₃(s) vaporized incongruently and predominantly as Rb₂ThO₃(s) = ThO₂(s) + 2Rb(g) + 1/2O₂(g). The equilibrium constant K = p²_Rb · p^1/2o₂ was evaluated from the measurement of the effusive flux due to Rb vapor species under the oxygen potential governed by the stoichiometric loss of the chemical component Rb₂O from the thorate phase. The Gibbs energy of formation of Rb₂ThO₃ derived from the measurement and other auxiliary data could be given by the equation, ΔfG°(Rb₂ThO₃,s) =–1794.7 + 0. 42T ± 5 . 0 kJ mol^–1 (1058 ≤ T = K ≤ 1187).