The adsorption behavior of ferrocyanide and ferricyanide toward aluminum oxide with a 0.01 M NaClO4 background electrolyte at 25 degrees C was investigated. Results obtained from different pH values demonstrated that the adsorption of these complex ions could be described by a modified Langmuir isotherm in the concentration range 0.5 X 10(-4) to 4 X 10(-4) M. This modified Langmuir isotherm developed here could accurately predict the equilibrium partition between solid and liquid phases while taking proton competition into account. Also, the apparent equilibrium constant (K-app) derived from the Langmuir equation for ferrocyanide or ferricyanide adsorption on aluminum oxide was compared with the intrinsic constant (K-int) calculated by triple-layer model simulation. This comparison revealed a strong correlation between the apparent equilibrium constant and the intrinsic constant. Moreover, in comparing K-app with K-int, we can infer that the adsorption of either ferrocyanide or ferricyanide complexes onto aluminum oxide is achieved through outer-sphere complexation.