Geometry and energy of the reactant (UO2(OH2)(5)center dot Cl+), the transition state (UO2(OH2)(5)center dot center dot center dot Cl+ double dagger), and the product (UO2Cl(OH2)(4)center dot OH2+) of the title reaction have been computed with complete active space SCF (geometries and vibrational frequencies) and multiconfiguration quasi-degenerate second-order perturbation theory (total energies). Hydration was treated using the polarizable continuum model. The two investigated active spaces, (12/11) and (12/12),produce the same results. In contrast to the water exchange reaction on UO2(OH2)(5)(2+), which proceeds via the associative (A) mechanism (which is a two step reaction involving an intermediate with an increased coordination number, UO2(OH2)(6)(2+)), water substitution by chloride follows the associative interchange (la) mechanism (which does not proceed via any intermediate). In this case, structure and imaginary mode of the transition state are not straightforward criteria for the attribution of the substitution mechanism, since they are both typical for the A pathway. The la mechanism was derived from the computed intrinsic reaction coordinate, which showed that no intermediate (for example UO2Cl(OH2)(5+)) exists as a local minimum on the potential energy surface. The activation free enthalpy is 31 kJ mol(-1). As for the water exchange reaction, the dissociative mechanism is unlikely to operate because of its higher free activation enthalpy (by approximate to 25 kJ mol(-1)).