Using thermodynamic perturbation theory the statistical mechanics of intermediate-valence rare earth compounds is investigated. The Gibbs' free energy is calculated to second order in the Anderson mixing interaction and is shown to diverge logarithmically as T to 0. At high temperatures the susceptibility is approximately of the Curie-Weiss form, chi =C/(T+T_F), where kT_f approximately Delta , the 'width parameter' associated with interconfigurational fluctuations.