Although the surfactant plays an important role in the preparation of a water-in-oil-in-water (W/O/W) emulsion, its concentration has been determined empirically. We investigated the location of a hydrophilic surfactant in a coarse W/O/W emulsion and a membrane-filtered fine emulsion. At first. appropriate concentrations of some hydrophilic surfactants to prepare a W/O/W emulsion were investigated from the viewpoints of the median diameter of the oil droplets and the encapsulation efficiency of a hydrophilic substance in the inner-water phase. The location of the surfactants and oil was examined for the W/O/W emulsions, prepared using polyglycerol monolaurate as a hydrophilic surfactant in the outer-phase solution at 1-10% (w/v) and octanoic acid triacylglycerol, which contained hexaglyceryl condensed ricinoleate as an oil-phase surfactant in the oil phase. The median diameter of the oil droplets in the W/O/W emulsion reached a minimum value at 3% (w/v) decaglycerol monolaurate (ML-750), and was constant over 4% (w/v). The excess decaglycerol monolaurate, that was not adsorbed on the oil-water interface was distributed to the outer-water and oil phases. The oil and hydrophobic surfactant were dissolved in the outer-phase solution at high concentrations of the hydrophilic Surfactant. These results suggest that the hydrophilic-surfactant concentration appropriate for preparing the W/O/W emulsion would be 3% (w/v) in the outer-phase solution and that the location of the components in the W/O/W emulsion would also be useful in estimating the adequate concentration of the surfactant. (C) 2004 Elsevier B.V. All rights reserved.