The stabilization of emulsions by a mixture of oppositely charged nanoparticles is investigated in relation to their behavior in water before emulsification. No emulsion can be prepared using either negatively or positively charged silica particles alone because the particles are too hydrophilic. Certain mixtures of the two particle types lead to heteroaggregation and a lowering of the net charge. Such mixtures, of increased hydrophobicity as verified by contact angle measurements, are capable of forming stable oil-in-water emulsions of excellent coalescence stability. The increased viscosity of the continuous phase also contributes to such stability.