[en] Requirements for applying the surface-complexation modeling approach to simulating radionuclide adsorption onto geologic materials are discussed. Accurate description of adsorption behavior requires that chemical properties of both adsorbent and adsorbate be characterized in conjunction with determinations of extent of adsorption. Critical chemical properties of adsorbents include dissolution and oxidation/reduction behavior, types and densities of adsorption sites, and interaction of sites with solution components. Important adsorbate properties include hydrolysis, complexation, oxidation/reduction, and oligomerization. Adsorption behavior is described by a set of chemical reactions and binding constants between: adsorption sites and solution components, adsorbate and solution components, and adsorbate and adsorption sites. Methods for implementing such an approach are discussed;examples based on solute adsorption onto oxides are presented. The approach currently used to simulate sorption onto geologic materials, i.e., the determination of distribution coefficients, yields estimates that are disparate and subject to large errors. Implementation of the surface-complexation modeling approach would greatly improve the predictability of the role of adsorption in regulating radionuclide transport in subsurface environments. Research efforts should be directed towards understanding radionuclide adsorption onto fixed-charge minerals (e.g., clays), carbonate minerals, and poly-mineralic assemblages representative of those present at potential repositories. 123 refs., 19 figs., 18 tabs