The hopanoid Lipids, bacteriohopanetetrol (tetrol) and phenyl acetate monoester bacteriohopanetetrol (pa-tetrol), were extracted from vesicles of Frankia (actinomycetes) and purified using solvent extraction and high performance liquid chromatography. Surface pressure versus specific area equilibrium isotherms of monolayers of tetrol, pa-tetrol, and mixtures of the two hopanoids were obtained. The collapse pressure for tetrol is about 36 mN m(-1), while for pa-tetrol it is about 18 mN m(-1). Pa-tetrol has a much more expanded isotherm than tetrol due to the presence of the phenyl acetate group at the interface. Mixtures of tetrol and pa-tetrol are miscible and show an area condensation effect that is most pronounced around 30-70 wt % pa-tetrol as determined hom the isotherm data. Langmuir-Blodgett films of tetrol and pa-tetrol were analyzed by Fourier transform infrared spectroscopy. Transmission and grazing-angle absorption-reflection spectra show that tetrol is oriented nearly perpendicular to the interface.