Enhanced mobility of radionuclides has been observed at several waste disposal sites; radioactive wastes have often contained strong, organic complexing agents as co-contaminants. Adsorption at mineral-water interfaces is one of the types of reactions by which radionuclides may become immobilized in the subsurface environment. For a contaminant metal introduced into the subsurface in organically-complexed form, such immobilization requires either adsorption of the intact complex or displacement of the contaminant metal from the complex. Comparison with complexation reactions in solution suggests that, in the tatter case, immobilization of the contaminant metal may be subject to kinetic limitations. A comparative discussion of the mechanisms of complexation reactions in solution and at surfaces serves as background for examination of laboratory studies of competitive- and co-adsorption and field studies of metal mobility.