We studied the structures UO2(CO3)(3)(4-), Ca2UO2(CO3)(3)(0) and Ba2UO2(CO3)(3)(0) using HF, MP2, and B3LYP methods. We obtained an unusual long uranyl bond for the UO2(CO3)(3)(4-) complex by ECP calculations. This study shows that it is crucial to explicitly include the counterion(s) and the hydration water molecules in the calculation of the aqueous uranyl tricarbonate complex, The calculated structure of the hydrated Ca2UO2(CO3)(3)(0) has good agreement with the experiment. (C) 2002 Published by Elsevier Science B.V.