We report the results of a calorimetric study on the hydrolysis of UO22+ in different ionic media (NaClO4aq, NaClaq) at 25 degreesC. Experiments in NaCl were performed at different ionic strength, at I less than or equal to 1 mol l(-1). The species considered in both ionic media were UO2(OH)(+), (UO2)(2)(OH)(2)(2+) and (UO2)(3)(OH)(5)(+), and in addition (UO2)(3)(OH)(4)(2+) and (UO2)(3)(OH)(7)(-) in NaClaq. The dependence on ionic strength of enthalpy changes in NaClaq was expressed by the simple linear equation DeltaH(pq) = DeltaH(pq)degrees + al(1/2) (a, empirical parameter). Comparison with literature findings is given and some recommended values are reported. (C) 2004 Elsevier B.V. All rights reserved.