The water exchange reaction of the neptunyl(VI) and -(V) aqua ions was investigated with quantum chemical methods. Associative ( a) and dissociative (d) exchange mechanisms were studied. The geometries and vibrational frequencies were computed with density functional theory (DFT) and multiconfiguration self-consistent field (MCSCF) methods. The Gibbs activation energies (Delta G(double dagger)) for a and d pathways were calculated with DFT and wave function theory (WFT), extended general multiconfiguration quasi-degenerate second-order perturbation theory (XGMC-QDPT2) including spin-orbit (SO) coupling at the SO configuration interaction (CI) level. The DFT-WFT agreement for Delta G(double dagger) was poor for the investigated functionals except for LC-BOP-LRD. Due to ligand-field effects, Delta G(double dagger) for the associatively activated exchange reaction of NpO2(OH2)(5)(2+) with a f(delta)(1) electron configuration is higher than for the actinyl(VI) aqua ions of U, Pu, and Am exhibiting f(0), f(delta)(2), and f(delta)(2)f(phi)(1) electron configurations.