The interaction of Np(VI) in 0.1 M NaHCO3 solution, distilled water, or artificial seawater with basalt or olivine results in the production of Np(V) and Np(IV). The rate of increase in Np(V) was determined over periods of 120 (olivine substrates) and 140 days (basalt substrates). The Np(V) that does not remain in solution is preferentially adsorbed on the surface of the minerals as compared to the surface of the containers. The mechanism for the production of Np(IV) and Np(V) postulates adsorption and desorption steps for the aquo Np(VI) and Np(V) ions on to the silicate surface. The electron transfer reaction takes place when the Np(VI) or Np(V) is adsorbed on this surface at an Fe(II) site. Reduced neptunium ions are then adsorbed from the solid.