Surface pressure (pi)-area (A), surface potential (DeltaV)-area, and dipole moment (mu (perpendicular to))-area isotherms were obtained for Langmuir monolayers made from (perfluorooctyl) pentylphosphocholine (F8C5PC), (perfluorooctyl) pentanol (F8C5OH), and their combinations. Monolayers were spread on a 0.15 M NaCl subphase. Surface potentials were analyzed using the three-layer model proposed by Demchak and Fort. The contributions of the CF3 omega -group and the phosphocholine (PC) headgroup to the vertical component of the dipole moment mu (perpendicular to) were estimated to be -2.5 and 3.3 D, respectively. The miscibility of F8C5PC and F8C5OH in the mixed monolayers was examined by plotting the variation of the molecular area, surface potential, and dipole moment as a function of the F8C5OH molar fraction, using the additivity rule. Negative deviations to the linearity of the molecular area were observed at various surface pressures (5, 15, 25, and 35 mN m(-1)), showing partial miscibility with attractive interaction between the monolayers components. Assuming a regular surface mixture, the Joos equation, which was used to describe the collapse pressure of mixed monolayers with miscible components, allowed calculation of the interaction parameter (xi = -0.52) and the interaction energy (210 J/mol) between the two fluorinated amphiphiles.