The water exchange reactions of [PuO2(OH2)(5)](2+) and [UO2(OH2)(5)](2+) were investigated with density functional theory (DFT) and wave function theory (WFT). Geometries and vibrational frequencies were calculated with DFT and CPCM hydration. The electronic energies were evaluated with general multiconfiguration quasi-degenerate second-order perturbation theory (GMC-QDPT2). Spin-orbit (SO) effects, computed with SO configuration interaction (SO-CI), are negligible. Both Actinyl(VI) ions react via an associative exchange mechanism, most likely I-a. The Gibbs activation energies (Delta G(double dagger)) at 25 degrees C are 33-34 and 30-37 kJ mol(-1) for [PuO2(OH2)(5)](2+) and [UO2(OH2)(5)](2+), respectively Delta G(double dagger) for dissociative mechanisms (D, I-d) is higher by more than 15 kJ mol(-1).