Perfluoropoly(oxypropylene), F((CF2)(3)O)(n)((CF2)(2))X, with-a hydrophilic head group (X = -COOH) has been investigated as a thin film spread onto the surface of water and as films transferred to solid substrates via the Langmuir-Blodgett technique. Between the onset of the isotherm and collapse, the polymer forms a continuous film of uniform thickness. The density of this film is the same as in the bulk polymer, and the film thickness depends on the area per molecule. The theory of tethered polymers predicts an entropic contribution to the free energy of the system due to deformation of the polymer coils. We propose a thermodynamic analysis in which we mathematically separate the contributions to the surface pressure from the head groups and from the stretching of the polymer chains. in addition, we investigate the influence of the subphase composition (aqueous solutions of CaCl2, poly(ethylenimine), HCl, FeCl3, NH3) and compare the isotherm with a model system for the isolated head group. Based on the thermodynamic analysis and the comparison of isotherms, we conclude that-in the case of a chain length; of approximately 100 sigma-bonds-there is no significant entropic contribution from the polymer chains.