Adsorption measurements of an anionic surfactant (p-n-sodium-octyl-benzenesulfonate) have been performed at the oxide solution boundary of zirconia suspensions. Three different isotherms have been obtained by varying the conditions of electrification of the oxide surface. The experimental isotherms are discussed in the light of literature data and analyzed on the basis of the Frumkin-Fowler-Guggenheim adsorption equation. Either attractive or repulsive lateral interaction terms are obtained depending on the oxide surface charge (and potential) and on the concentration of the base electrolyte.