To better understand the phase behavior of a pseudoceramide (SLE), a potential skin moisturizer and/or a drug carrier, we investigated the lipid-lipid interaction between SLE and a sterol-combined fatty acid (CEOS), which has a sterol ring and a carboxyl group in a molecule. X-ray analysis showed that a hexagonal packing (4.15 Angstrom spacing) and a liquid-like packing (4.5 Angstrom spacing) coexisted within the hydrocarbon chains of the SLE/CEOS (1/1 mole) lipid mixture. The structural characteristics were very similar to those of the SLE/stearic acid/cholesterol (1/1/1 mole) system, which was in a stable lamellar alpha-phase. However, in the SLE/stearic acid (1/1 mole) system, there was only a strong hexagonal reflection in the nide-angle X-ray profile. The melting enthalpy (23.9 kJ mol(-1)) and entropy (75.0 J mol(-1) K-1) of the SLE/CEOS system were also smaller than those(Delta H-m, = 43.9 kJ mol(-1) Delta S-m, = 131.6 J mol(-1) K-1) of the SLE/stearic acid system. The X-ray data along with the DSC results suggested that the sterol ring of CEOS molecule contributed to the enhancement of molecular motion or the decrease in the molecular packing of lipids. A strong hydrogen bond between the carboxyl group of CEOS and the amide group of SLE molecule was also considered to be important for the formation of the stable alpha-phase, as suggested by FT-IR spectroscopy. Further, in the presence of water, the three artificial SC lipids, SLE/CEOS(1/1 mote), SLE/stearic acid/cholesterol(1/1/1 mole), and SLE/stearic acid (1/1 mole), were all capable of forming lamellar structures.