Abstract Adsorbed amount of sodium dodecylsulfate (SDS) and n-dodecanol (DOH) at the gas-solution interface of their mixed aqueous solution was directly measured as a function of bulk concentration of DOH by the radiotracer method. For this purpose the surface radioactivity of the mixed solution in which either SDS or DOH was labeled with tritium, was counted by using a 2π window-less gas-flow counter. It was found that the adsorbed amount of DOH rapidly increases with increasing bulk concentration of DOH, while that of SDS gradually decreases. The adsorption of DOH and desorption of SDS are not a simple 1 : 1 replacement. A maximum total adsorption of about 13×10−10 mol/cm2, which exceeds the amount of close-packed monolayer adsorption extends over a wide range of concentration studied. It was suggested that the adsorbed film consists of mixed monolayer or double layers. The latter film is composed of mixed monolyaer of SDS and DOH with adsorbed layer of SDS beneath. The molar ratio of adsorbed amount of DOH to SDS in the surface became unity, when the ratio of the bulk concentration of DOH to SDS was about one hundredth, which shows a stronger adsorption tendency of DOH than that of SDS.