Surface pressure (pi)- and surface potential (Delta V)-area (A) isotherms were obtained for monolayers of alkylammonium perfluorohexanoates of four different alkyl chains (m) on water and 4.4 M NaCl solutions as a function of molecular area of various temperatures by the Langmuir and the ionizing electrode methods, As for octadecylammonium perfluorohexanoate (C18F11), the pi-A curves of these long-hydro-short-fluoro chain salt showed three phase transition points, Judging from the surface potential and the apparent molar quantity together with our previous data on the transitions, the first was assigned to the transition from the expanded (E) phase to the condensed (C-I) phase, the second from the C-I phase to another condensed (C-I') phase, and the third from the (C-I') phase to another condensed (C-II) phase, From the surface potential, the apparent molar entropy change, and the phase diagram, the orientation of long-hydro-short-fluoro chain salts in the monolayer state was discussed. (C) 1996 Academic Press, Inc.