This study aims to elucidate the working mechanism Df oleic acid (OA) on isolated human stratum corneum (SC) sheets using two in vitro techniques, differential thermal analysis and freeze-fracture electron microscopy. Differential thermal analysis on SC after the application of OA in propylene glycol revealed significant changes in the thermal profiles of SC compared to that of the untreated SC. The changes occurred generally on the lipid phase transitions by both shifting the temperatures to a lower degree and reducing the enthalpies of the transitions normally observed between 40 degrees and 90 degrees C. Another newly observed change took place in the temperature range below O degrees C, referred to as the subzero region. The subzero transition of OA has profoundly influenced the subzero SC lipid transition (normally observed at around -10 degrees C) by shifting it to a lower temperature. The interesting observation was that the subzero transition of SC lipid and of OA became a single transition after the SC is heated to 120 degrees C, which indicates a close interaction between oleic acid and SC lipids. Electron micrographs obtained by freeze-fracture electron microscopy revealed the formation of a new structure in the intercellular lipid regions of SC in the presence of OA. These findings may prove that oleic acid acts as a skin penetration enhancer by forming together with SC lipid a new type of lipid domain which are responsible for the decreased capacity of skin barrier function after oleic acid treatment.