The structure and vibrational frequencies of equatorially coordinated uranyl complexes have been determined by using ab initio Hartree-Fock methods including effective core potentials. It was found out that the hydration of the uranyl is not negligible and should be included in the calculation to properly comprehend the chemical properties. The correlation was found between the theoretical and the experimental vibrational frequencies by taking the hydration effect into account. (C) 1999 Elsevier Science B.V. All rights reserved.