Ab initio methods were applied for the calculation of the total energy and the molar entropy and heat capacity of the compounds UO2F2, UO2(OH)(2), UF6, and UO3 in the gas phase with the purpose to obtain thermodynamic data for reactions that can be compared with experimental values. The total energy, geometry, and vibration frequencies were calculated at different levels of accuracy: second-order perturbation theory (MP2), coupled cluster theory (CCSD(T)), and density functional theory (B3LYP). Our results agree well with experimental values and previous theoretical results. Additionally, the transition state Of UO2F2(g) was studied and the value of the barrier for the inversion of the fluoride atoms was calculated.