Fluorinated surfactants have been studied for their peculiar property to form micellar aggregates in water and oils (hydrocarbons or fluorocarbons) and to produce stable microemulsions. Because of their capacity to dissolve large amounts of gases (such as oxygen and carbon dioxide) and for their characteristic physicochemical properties, fluorocarbons have been tested for specific medical purposes, and their microemulsions are among the most promising candidates for the production of suitable blood substitutes and other biocompatible fluids. We have synthesized a new partially fluorinated nonionic surfactant, namely, F(CF2)7-CO-(OCH2CH2)7.2- OCH3 (I), that forms stable microemulsions with water and perfluorocarbons such as perfluorooctane (PFO). In this paper we describe for the first time the phase behaviors of perfluorooctanoic acid (PFOA) in water/ PFH and in water/PFO, and that of ester I in water/PFO. Small-angle neutron-scattering (SANS) experiments provide a detailed description of the microstructure of the H2O/PFO/PFOA ternary system.