The solution behavior of the fluorinated surfactant tetraethylammonium perfluorooctane-sulfonate, PFOS, in water-trifluoroethanol, TFE, mixtures has been investigated by surface tension, electrical conductance, and PGSE (pulsed gradient spin-echo) NMR self-diffusion methods. Addition of progressive amounts of TFE in the solvent has little influence on the critical micellar concentration, cmc. Conversely, self-diffusion, counterion binding, and the surface pressure at the cmc are significantly affected by added fluoroalkanol. The above effects have been explained in terms of the solvent viscosity dielectric permittivity, and surface activity, respectively. The complete phase behavior of the above system has been drawn, and the phase boundaries were determined. According to the above findings, added surfactant promotes the separation of the homogeneous solvent mixture into two coexisting fluid phases. The observed behavior was rationalized on thermodynamic grounds.