Investigation of reactions between neptunium and soil samples representative of the saline area around the Gorleben salt dome (Federal Republic of Germany) was conducted to obtain an understanding of the transport mechanism of neptunium in saturated brine aquifers. Leaching of 237Np-doped glasses with brine under oxic conditions resulted in the release of soluble species of Np(V). Adsorption parameters obtained from the application of nonlinear sorption isotherms to static experiments were used to interpret the migration of neptunium through soil columns. The existence of two different adsorption sites reacting with neptunium at different rates was postulated. Retardation factors under oxic and anoxic conditions were measured. In anoxic environments such as those found in undisturbed repository horizons, more neptunium activity was fixed by the soil.