Abstract The interfacial properties of surfactin were studied. The adsorption at the air-water interface was detectable at 5 × 10 −8 M. The compression isotherms of surfactin films have been recorded as a function of the pH subphase and the p K at the interface determined (p K ⋍ 6.5). In mixed lipid monolayers ionized surfactin ismiscible with DMPC. When surfactin is protonated, the mean area-composition plots followed the additivity rule. In mixed surfactin/cholosterol the mean are plots show positive deviations suggesting the formation of bidimensional patches.