A W/O/W emulsion containing octanoic acid triacylglycerol (C8TG) was prepared as a carrier for hydrophilic bioactive substances. The absorption enhancement effect of the hydrolysates of C8TG in the W/O/W emulsion was investigated by measuring the transport of a hydrophilic marker through the intestinal epithelial (Caco-2) cell monolayer. First, the concentrations of gall and lipase in an artificial digestive fluid, of which the integrity of the Caco-2 monolayer was not affected, were both determined to be 0.1% (w/v). The W/O/W emulsion including the hydrophilic marker molecule was hydrolyzed by the artificial digestive fluid and the apparent permeability of the hydrophilic marker substance was measured. In order to clarify the components affecting the transport enhancement, octanoic acid monoacylglycerol (C8MG) was mixed with a W/O/W emulsion at the same concentration as that in the emulsion hydrolyzed by the digestive fluid. The dependence of the apparent permeability on the C8MG concentration was the same for both emulsions, indicating that C8MG, hydrolyzed from the oil phase of the W/O/W emulsions, played an important role in the transport enhancement effect of the W/O/W emulsions. (C) 2004 Elsevier Ltd. All rights reserved.