Time-resolved fluorescence and differential scanning calorimetry were used to determine the partitioning of coumarin 152 (C152) into large unilarriellar vesicles composed of binary mixtures of two phosphatidylcholines (12:0/12:0 DLPC and 14:0/14:0 DMPC) and vesicles composed of binary mixtures of a phosphatidylcholine and a phosphatidylethanolamine (14:0/14:0 DMPC and 14:0/14:0 DMPE). Differential scanning calorimetry showed that both DLPC/DMPC and DMPC/DMPE are miscible in lipid vesicles. Timeresolved fluorescence indicated that C152 partitioning into DLPC/DMPC mixtures showed nearly ideal behavior that was described With weighted contributions from,C152 partitioning into pure DLPC and pure DMPC vesicles.,In contrast, C152 partitioning into DMPC/DMPE mixtures was distinctly nonideal. For DMPC/DMPE lipid vesicles having DMPC mole fractions between 10 and 80%, C152 partitioning into the bilayer was measurably enhanced near the melting temperature, relative to expectations based simply on weighted contributions from C152 partitioning into vesicles comprised of pure lipids. The origin of this behavior remains uncertain. For vesicles comprised of pure DMPE, C152 shows almost no partitioning into the membrane, with >= 80% of the solute remaining in the buffer solution at temperatures between 10 and 50 degrees C.