The effects of core triglyceride (TG) on the surface properties of lipid emulsions were investigated. Fluorescence lifetime measurements showed that despite the similar surface composition and curvature, surface phosphatidylcholine (PC) monolayers of emulsions are more hydrated than those of bilayer vesicles. Medium-chain TG had less influence on the membrane surface hydration than long-chain TG. Differential scanning calorimetry revealed a reduced transition enthalpy (DeltaH = 3.3 kcal/mol) of dipalmitoyl acyl chains of emulsion surface PC compared to PCbilayers (DeltaH = 5.6 kcal/mol). Fluorescence anisotropy measurements demonstrated that the presence of core lipids brought about the condensation of the acyl chains in emulsion surface PC monolayers, whereas the incorporation of TG into the PC bilayer surface had little effect on the acyl chain order. Likewise, lateral diffusion of surface PC was strongly restricted in the emulsion surface compared to bilayer vesicles, and such an effect of core TG depended on the acyl chain length. Taken together, these observations suggest that the interpenetration of acyl chains between surface and core lipids appears to modulate the surface properties of lipid emulsions; the interpenetration of acyl chains in emulsion causes the headgroup separation of the surface PC so as to make space for apolipoprotein binding as previously suggested (Saito, H., et al. Langmuir 2001, 17, 2528-2532).