Undecylglycerylether-modified silicone (GES; the glycerylether-type surfactant with a silicone segment and alkyl chains (carbon number, 11) as the hydrophobic portion) forms a molecular aggregate (M.A.) with a small amount of water. This M.A. is similar to the reversed hexagonal liquid crystal formed by alpha-mono long-chain alkylglycerylether (3-isooctadecyloxy-1,2-propanediol; GE). From the investigation of the phase behavior in the water/GES/polydimethylsiloxane (PDMS) ternary system, a wide three-phase region of water (W)+M.A.+oil (O) was observed. As this M.A. is insoluble in PDMS and easily orients in the interface between water and PDMS, the high water content silicone W/O emulsion using GES as a surfactant is well stabilized. However, as the PDMS content increased this W/O emulsion became less stable. In order to improve this stability, mixtures of GES and polyoxyethylene-modified silicone (PS) were applied to the silicone emulsion as co surfactant. By application of a PS with a methyl group at the end cap of the polyoxyethylene chain (PSM), the emulsion became most stable at a GES/PSM ratio of 1 : 2, and at the same time, the interfacial tension between the oil phase and the water phase became minimal. The reason for this was studied by the measurement of spin-lattice relaxation times (T(1)) of the alkyl chains of GES in the GES/PS/water system by (13)C NMR. We assumed that the W/O silicone emulsions were stabilized by the efficient orientation of the aggregates in the interface between the silicone phase and the water phase by using PSM as a cosurfactant. Copyright 2001 Academic Press.