Humic acid (HA) was extracted from the surface soil of an evergreen broadleaf forest in the Jingyun Mountain in China.Characteristics of humic acid adsorbing and desorbing Fe³⁺ ions under conditions varying in pH, ion strength and temperature were studied with the C-25 glucosan gelchromatography method.The results show that the max adsorption S_max and adsorption equilibrium constant k increased with pH, and the thermodynamic standard molar free energy change ΔG_m^o decreased, when ion strength and temperature remained the same.With pH and temperature unchanged, S_max and k increased and ΔG_m^o decreased when ion strength rose from zero to 0.10 mol L^-1, but things all went in reverse when ion strength continued to rise to 0.15 mol L^-1.When temperature increased, S_max, k, and ΔG_m^o all decreased significantly, indicating that higher temperature was disadvantageous to adsorption reaction.The thermodynamic standard enthalpy change ΔH_m^o and entropy change ΔS_m^o of adsorption reaction were both below zero, suggesting that adsorption reaction was an exothermic one and tended to be more in ordered state.When the other factors remained the same, the higher the pH, the higher the absolute values of ΔH_m^o and ΔS_m^o, suggesting that higher pH was advantageous to the process of adsorption reaction.The percent reduction of Fe³⁺ ions decreased with increasing pH, which fit the power function equation for description, showing a correlation coefficient reached the level of significance.The desorption rate of HA-Fe³⁺ increased when pH decreased, showing a curve that fit the linear equation, with a correlation coefficient also reaching the level of significance.Obviously, the Fe³⁺ adsorption reaction was a "two-phases" reaction, encompassing interior and exterior reactions of humic acid.