A molecular dynamics simulation method for the first hydration shell exchange kinetics is described and is illustrated for Na+ in water. The exchange process is found to have a complex character. A calculated small transmission coefficient, K approximate to 0.21, indicates that transition-state theory seriously overestimates the rate. Grote-Hynes theory is unsuccessful in accounting for the kappa value. This is traced to trajectory mechanistic aspects for which the theory’s assumptions break down. Inclusion of anharmonicity via the theory of Voth provides an improved estimate.