The adsorption isotherms of sodium perfluorooctanoate and sodium decyl sulfate and their 1:1 mixture on gamma-alumina are recorded by depletion-type experiments with H-1 and F-19 NMR spectroscopy as the detection tool. The isotherms of the different surfactant species, obtained with and without added salt, closely resemble each other. Salt addition changes the isotherms from stepwise to the familiar S-shaped. After having reached saturation, a further increase of surfactant concentration in the mixed system leads to decyl sulfate desorption and increased perfluorooctanoate adsorption. The F-19 chemical shift of adsorbed perfluorooctanoate suggests that, for saturated surfaces, the two sorts of adsorbed surfactants form molecularly mixed surface aggregates.