The water exchange reaction of the americyl(VI) aqua ion was investigated with quantum chemical methods, density functional theory (DFT), and wave function theory (WFT). Associative and dissociative substitution mechanisms were studied, whereby DFT produced inaccurate results for the associative mechanism in contrast to WFT. The Gibbs activation energies (Delta G(double dagger)) for the dissociative (D) and the associative interchange (I-a) mechanisms, computed with WFT taking into account static and dynamic electron correlation, near-degeneracy, and spin orbit coupling, are equal within the error limits of the calculations. Delta G(double dagger) for the water exchange of americyl(VI) via the dissociative mechanism is considerably lower than those for uranyl(VI) and plutonyl(VI) (for which the I-a mechanism is preferred) due to ligand-field effects. On the basis of the present computations, it is not possible to distinguish the I-a from the D mechanism for americyl(VI). In contrast to two other theoretical studies, the dissociative mechanism cannot be ruled out.