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 aniomic 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 aniomic dodecyl sulfate with Cs⁺ ions through the addition of Cs⁺ ions into water. This subsitution 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.