Surfactin, a lipopeptide produced by Bacillus subtilis, is characterized by a lactone bond. — Interfacial properties of surfactin were studied at w/a interface: spreading isotherms at 17.5o and 20.5°C were recorded as a function of subphase composition (water at two different acid pH, mono and divalent ions solutions at the same pH and ionic strength). — The isotherms were affected by temperature and subphase composition, bo th in the shape and in the limiting area, although the films remain always in the liquid condepensed phase. — The monolayers were then transferred onto a solid substrate according to the Langmuir-Blodgett technique: the ellipsometric measurements confirm the planar disposition at the w/a interface and the FTIR spectra show the presence of the lactone ring also in the bidimensional state of molecule. — Furthermore, mixed monolayers of surfactin with methylstearate, which is very stable in bidimensional state, were studied at the w/a interface.