The outer part of the skin, stratum corneum, is essential to the skin's barrier function. Monolayer and bulk phase behavior of stratum corneum model lipids have thus been studied. Domain formation in Langmuir-Blodgett monolayers of synthetic ceramides (C16CerIII and C24CerIII), cholesterol, and free fatty acids (Lignoceric acid, C24:0, and palmitic acid, C16:0) were investigated by atomic force microscopy. Binary, ternary, and more complex lipid mixtures were examined. It was shown that small amounts of ceramide are miscible in a cholesterol-rich phase with the miscibility dependent upon the ceramide chain length. Two phases are formed at low and intermediate cholesterol concentrations. In the ceramide-cholesterol monolayers, very small rectangular-shaped ceramide domains thought to be two-dimensional single ceramide crystals are formed. Small domains were also found in more complex mixtures where the fatty acid is miscible in the ceramide phase, although these domains were not as regular in shape. Binary ceramide-cholesterol as well. as ternary ceramide-cholesterol-lignoceric acid bulk mixtures were also studied by X-ray diffraction. The bulk Lipid miscibility is consistent with the monolayer results.