Ceramides, the major lipid component of the stratum corneum (SC), provide many of its unique physical properties. Surprisingly, only a few biophysical studies of hydrated ceramides have been reported. The current Fourier transform infrared (FTIR) spectroscopy investigation provides the first detailed study of intermolecular and intramolecular chain and headgroup interactions in hydrated non-hydroxy fatty acid (NFA) and hydroxy fatty acid (HFA) ceramides. Information about NFA and HFA ceramide chain subcell structure and conformational order is derived from the temperature dependence of the methylene stretching, scissoring, and rocking mode frequencies. At low temperatures, NFA ceramide is highly ordered and packed in an orthorhombic subcell structure that undergoes a solid-solid phase transition to a conformationally ordered hexagonal phase at 60 degrees C. A second transition to a conformationally disordered non-bilayer-like phase occurs at 80 degrees C. The lipid chains of HFA ceramide undergo a single transition from a conformationally ordered, orthorhombic subcell, phase to a conformationally disordered phase at 76 degrees C. In NFA ceramide the amide I and II modes of the headgroup are each split into two bands, indicating strong intermolecular headgroup coupling between two NFA ceramide headgroups in a factor group perpendicular to the bilayer plane. In contrast, splitting is not observed for either amide mode in HFA ceramide. However, the presence of strong H bonding indicates an interaction between molecules in the bilayer plane. The contrasting behavior of the headgroups in NFA and HFA ceramide suggests that these molecules make distinct contributions to the structural integrity of the stratum corneum. The implications of these findings to the recently proposed domain mosaic model of the stratum corneum lipid barrier are discussed.