Abstract In the past 3 decades, research has proven the significance of competitive adsorption in the equilibrium of pollutants between solid and liquid phases. However, studies on the competitive adsorption of complex ions are very limited in spite of its important role in transporting pollutants in the natural environment. The objective of this study is to derive the thermodynamic parameters of the competitive adsorption between ferricyanide and ferrocyanide from the modified Langmuir isotherm and the triple-layer model (TLM) to determine the location of adsorption. The effects of pH, temperature, and ion concentration on competitive adsorption onto γ-Al2O3 were investigated. The results demonstrate that ferrocyanide is more competitive than ferricyanide. By comparing the derived Kapp with Kint, we inferred that the adsorption of ferricyanide and ferrocyanide onto γ-Al2O3 was achieved through outer-sphere complexation. The negative ΔH° indicated that the adsorption was exothermic. The positive entropy (Δ S°) was caused by the replacement and release of a greater number of smaller surface ions by adsorbed ferricyanide and ferrocyanide ions of larger size.