The interfacial behavior of water-soluble poly[oligo(ethylene oxide) monomethyl ether methacrylate], grafted from 15-nm-diameter silica nanoparticles using a living atom-transfer radical polymerization technique, was examined for hexane and water interfaces. The polymer-grafted nanoparticles reduced the hexanewater interfacial tension from similar to 50 to similar to 20 mN/m at concentrations of silica in the range of 110 ppm. The hydrodynamic size of the dispersed hybrid nanoparticle, a function of the molecular weight of the polymer and grafting density, was the dominant variable in determining the critical particle concentration and efficacy of the hybrid nanoparticles in reducing the hexanewater interfacial tension. A simple phenomenological model is used to explain the strong dependence of the critical particle concentration on the effective hydrodynamic size of the nanoparticles. Waterhexane and watersqualene emulsions formed using 1000 ppm of hybrid nanoparticles were stable for more than 60 days.