The effects of non-ionic surfactants added to the oil phase and to the outside water phase, on the oil droplet diameters in O/W emulsions and on the W/O emulsion drop diameters in W/O/W emulsions, respectively, were investigated experimentally. Sauter mean diameters of the oil droplets in O/W emulsions, and of the W/O emulsion drops in W/O/W emulsions, had minimum values at a specific concentration of surfactant in the outside aqueous phase. The effect of the surfactant concentration on the drop diameter corresponded to that of the interfacial stretching forces which were measured with the aid of a ring method using a Du Nouy type platinum ring.