The adsorption of a nonionic surfactant (Triton X-100) from aqueous solutions has been studied on sixcarbon blacks with very different specific surface areas. The surface area occupied per surfactant moleculeon the carbon black surface and the free energy of adsorption have been evaluated from the adsorptionisotherms. Also, the free energy of adsorption has been determined from the free energy of interactionbetween adsorbent and adsorbate through water. The results obtained from both methods are in goodagreement. They indicate that adsorption progresses following two different processes: the first one dealswith the direct interaction between carbon black surface and adsorbate molecules, and the second onemainly due to the interaction between surfactant molecules at the adsorbent−solution interphase.