The adsorption isotherm of sodium octylbenzene sulfonate has been determined at two pH values on dispersed maghemite particles in water. The isotherms present an adsorption maximum in the region of the surfactant critical micelle concentration. Additional adsorption experiments at the alumina/water interface as well as at the air/water interface in the presence of ferric ions suggest that the adsorption maximum in the case of the iron oxide particles is due to the presence of very small concentrations of ferric ions in the aqueous solution. These ions form surface active complexes with sodium octylbenzenesulfonate. These complexes are adsolubilized in the surfactant layers on the particles below the critical micelle concentration. They are desorbed from the surface and transferred into free micelles above the critical micelle concentration. These two phenomena are shown to be responsible for the adsorption maximum observed. The presence of ferric ions induces a surface tension minimum of the anionic surfactant at the air/water interface. This effect is discussed in relation to the adsorption maximum at the solid/water interface,