Partitioning of coumarin 152 (C152) in phosphatidylcholine vesicles was quantified using time-resolved fluorescence emission. Phospholipid vesicles were comprised of 1,2-dilauroyl-sn,glycero-3-phosphocholine (12:0 DLPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (14:0 DMPC), and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (16:0 DPPC). C152 C152 fluorescence emission decays were fit to three lifetimes, corresponding to C152 solvated by the aqueous buffer, embedded in polar lipid headgroups, and surrounded by the nonpolar lipid membrane core. Partitioning was measured as a function of sample temperature and vesicle "composition. C152 in all three lipid systems showed qualitatively similar partitioning behavior. Partitioning into a gel phase membrane was thermoneutral and slightly entropically Partitioning of C152 neat the lipid membrane headgroups was entropically driven and endothermic. Well above the melting temperatureY'exsolvation of C152 from the:membrane back into the aqueous buffet was, enthalpically driven but entropically unfavorable Regardless of solution temperature, relatively little (<15.%),C152 partitions into the hydrophobic core of the membrane. The magnitudes of the 'forces driving C152 partitioning systematically increased with alkyl chain length (DLPC < DMPC < DPPC). Differences in partitioning between the three vesicle systems are attributed to differences in surface area per lipid as membrane phase changes froin the gel to liquid crystalline state.