Silica surfaces were premodified by the saturated adsorption of the cationic polyelectrolyte of poly(2-vinyl-1-methyl-pyridinium bromide), P2VP, in water. The interaction forces between and the zeta potential of the silica surfaces were then measured in water and in ethanol solutions of perfluorinated anionic surfactants as a function of their chain length and concentration using the AFM surface force and electrophoresis methods. In water, the electrostatic repulsive forces between P2VP-modified silica surfaces in CF3CF2COONa, CF3(CF2)(6)COONa, and CF3(CF2)(7)SO3Li solutions of 0.1 mM were identical to the force curve in 0.1 mM NaNO3 but greatly decreased in 0.1 mM CF3(CF2)(9)COOLi (critical micelle concentration (cmc), 0.39 mM). The concentration increase of CF3(CF2)(7)SO3Li (cmc, 6.3 mM) from 0.1 to 1.0 mM caused the repulsive force curves to weaken and then to strengthen after passing through a zero repulsive force. The surface potentials obtained by the best curve fitting of the force curves agreed well with the zeta potentials, which indicated a surface charge reversal from positive to negative for a high concentration of CF3(CF2)(7)SO3Li. These observations were explained by the formation of a Stern layer due to specific counterion binding of the surfactant anions, which increased with the surfactant chain length and concentration. In ethanol, CF3(CF2)(7)SO3Li always showed strong repulsive force curves, when CF3(CF2)(7)SO3Li concentrations that were adjusted to give identical Debye lengths as those in water were used and the surface charge of P2VP-modified silica was the same as that in water. The surface potential obtained by the best curve fitting coincided with the zeta potential of positive sign, confirming no charge reversal. This suggested no obviously firm formation of a Stern layer by the surfactant ions. This was not always the case for shorter carbon-chain surfactants, since CF3(CF2)(6)COONa revealed a much weaker repulsive force curve than CF3(CF2)(7)SO3Li for concentrations of identical Debye lengths. This was explained in terms of an increased surfactant binding, due to the polarity difference between the solvent and surfactant molecules.