Reduction of the interfacial tension at the sodium dodecyl sulfate (SDS) solution/ hexadecane interface by mixing with cesium dodecyl sulfate and adding NaCl or CsCl into the aqueous phase was investigated using an interfacial tensiometer. The degree of binding of hydrated Cs^+ ions to anionic dodecyl sulfate at oil/water interface appeared higher than that of Na^+ ions. The higher binding capacity of Cs^+ than Na^+ resulted in the substitution of the Na^+ bound to anionic dodecyl sulfate with Cs^+ ions through the addition of Cs^+ ions into water. This substitution reduced the interfacial tension of SDS solution since the hydrated ionic size of Cs^+ is smaller than Na^+, and the electrostatic repulsive force between Cs^+ ions bound to anionic dodecyl sulfate at interface was weaker than that between Na^+ ions. The reduction in interfacial tension of SDS solution by NaCl was smaller than that by CsDS and CsCl.