In order to characterize the surface properties of cytochrome f (cyt f), the study of the surface pressure-area and surface potential-area isotherms of cyt f monolayer spread at air-water interface has been carried out. The stability of the cyt f monolayer has been found to be highly dependent on various experimental parameters, e.g. the interfacial concentration of cyt f, the spreading solvent and the pH of the subphase. These observations have been explained in terms of the conformational changes in cyt f and its solubilization in the subphase.