Technical Abstract: The chemical processes and soil factors that affect the concentrations of As and Se in soil solution were discussed. Both elements occur in two redox states differing in toxicity and reactivity. Methylation and volatilization reactions occur in soils and can act as detoxification pathways. Precipitation-dissolution reactions control As and Se concentrations only in highly contaminated situations. Oxidation-reduction reactions determine which redox state is thermodynamically stable, although more than one redox state may be present in soils. Adsorption-desorption reactions most often control the dissolved As and Se concentrations. Empirical and chemical models of adsorption were discussed. The constant capacitance model, a chemical surface complexation model, has been used to predict As and Se adsorption under changing conditions of solution pH and equilibrium ion concentration.