Electronic structure calculations at the coupled cluster (CCSD(T)) and density functional theory levels with relativistic effective core potentials and large basis sets were used to predict the isolated uranyl ion frequencies. The effects of anharmonicity and spin-orbit corrections on the harmonic frequencies were calculated. The anharmonic effects are larger than the spin-orbit corrections, but both are small. The anharmonic effects decreased all the frequencies, whereas the spin-orbit corrections increased the stretches and decreased the bend. Overall, these two corrections decreased the harmonic asymmetric stretch frequency by 6 cm(-1), the symmetric stretch by 3 cm(-1), and the bend by 3 cm(-1). The best calculated values for UO22+ for the asymmetric stretch, symmetric stretch, and bend were 1113, 1032, and 1.74 cm(-1), respectively. The separation between the asymmetric and the symmetric stretch band origins was predicted to be 81 cm(-1), which is consistent with experimental trends for substituted uranyls in solution and in the solid state. The anharmonic vibrational frequencies of the isoelectronic ThO2 molecule also were calculated and compared to experiment to calibrate the UO22+ results.