Interfacial events during lipid hydrolysis by cutinase are described as measured with the oil-drop tensiometer. A linear relation between enzyme concentration and initial decrease of oil-water interface tension (γ O / W ) due to lipolytic activity was observed. The amount of hydrolysis products showed a non-linear relation with γ O / W . Hydrolysis is linear with time, even when the area occupied by the fatty acid molecules exceeds the drop surface by a factor 7000. At pH 9.0, fatty acids were found to partition mainly in the oil phase. Formation of calcium soaps and ionization increase the impact of fatty acids on γ O / W without affecting enzyme activity. The presence of fatty acids at the interface, added prior to cutinase, delayed hydrolysis effects on γ O / W . Fatty acids in the water phase almost completely abolished adsorption effects on γ O / W , when the concentration was over the critical micellar concentration (cmc).