The present paper describes a theoretical evaluation of the influence of complexation on metal ion (e.g., radionuclide) migration in environments containing an excess of complexing agents, that is the equilibrium between the free and the complexed metal ionscan be regarded to follow pseudo first-order kinetics. It is shown that as long as the rate of interconversion between the free and complexed metal ions is rapid relative to the residence time in the system studied, the two species will migrate with the same speed, controlled by an"effective retention factor." It is clearly demonstrated that approaching zero complexation the effective retention approaches that of the more retarded species (the free metal ion), whereas the effective retention approaches that of the less retarded species (the complex) for increased complexation. The implication for the distribution of radionuclides in the terrestrial environment is discussed.