The effects of the initial pH and buffer on the adsorption isotherms for cupric and cadmium ions by corncob particles were investigated. The uptakes of the two ions on corncob particles increased with the initial pH. A modified Langmuir model, which considers the effect of pH on the isotherm, has been developed and was used to fit the experimental equilibrium data. The pH at zero-charge point of corncob particles was measured (pH(zcp) 4.97) by applying the primary equilibrium method of H+ and OH-. A method to determine the chemical free energy in the James-Healy model has been proposed in which the pH,,p is used to distinguish the chemical free energy from the solvation and coulombic free energies in that model. The calculated results indicate that the chemical and solvation free energies contribute much to the adsorption free energy. The chemical free energy is constant, while the solvation free energy increases with increasing pH; their effects are opposite. The adsorption free energy increases with the pH value during the adsorption processes of cupric and cadmium ions on corncob particles.