Counterions in dodecyl sulfate surfactants have a significant effects on basic surface chemical properties such as cmc, surface or interfacial tension, adsorption into air/water interface, followed by the applications of these surfactants in technological processes. In this study, the cmc and surface tension of sodium dodecyl sulfate and lithium dodecyl sulfate were measured in aqueous solution in the presence of LiCl, NaCl and CsCl. And these results were explained on the basis of difference in the hydrated counter-ion sizes.