The interactions of surfactants with the enzyme Rhizomucor miehei lipase were studied using surface tension measurements and ellipsometry. From earlier surface tension studies it is known that the cationic surfactant didodecyldimethylammonium bromide forms an aggregate with R. miehei lipase, while anionic and nonionic surfactants do not form such a complex. In this work the influence of surfactant charge on complex formation with the lipase is systematically investigated, using the amphoteric surfactant sodium N-(2-hydroxydodecyl)sarcosinate, The charge of the amphoteric surfactant was varied from anionic via zwitterionic to cationic by adjusting the pH. By this procedure the influence of the surfactant charge on the interaction with the enzyme could be studied while the molecular structure of the surfactant hydrophobe was kept unchanged. It was confirmed that only when the head group is positively charged does the enzyme bind with the surfactant. However, the number of surfactant molecules that bind to each enzyme is dependent on the surfactant structure. A tentative interaction model is presented, Below the critical micelle concentration binding occurs with negatively charged sites on the enzyme interacting with cationic surfactant head groups. It is likely that a hydrophobic domain adjacent to the negatively charged site of the enzyme enables a hydrophobic attraction between the tail of the cationic surfactant and the enzyme.