The goal of the present paper concerns the effects of counterions on adsorption behavior of anionic surfactants. The surfactants chosen are sodium dodecyl sulfate and cesium dodecyl sulfate, and the polar solvent is formamide. In the investigations, the surface excess and surface tension have been measured independently. On this basis the chemical potential was determined with the help of the Gibbs equation. In addition, the structure of electrical double layer has been investigated. It was determined by the distribution of the oppositely charged ions near the surface. The results evidence that the counterions have profound effects on the adsorption behavior of ionic surfactants on solution surface. The cause can be attributed to the properties of counterions, such as size and solvophilicity.