Although the surface adsorption of ionic surfactants had been widely studied,the investigations were almost limited to solutions containing only a single surfactant. In the present investigation surface tension of the mixed solutions containing a cationic and an anionic surfactant namely, dodecylpyridinium bromide and sodium dodecylsulfate, has been measured by the drop-volume method, and the individual adsorption of each surface-active ion calculated by the following equations.