Time resolved fluorescence emission was used to quantify coumarin 152 (C152) partitioning into a model lipid vesicle membrane. For these studies, the lipid vesicles were composed of the symmetric, saturated phosphocholine, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (14:0 DMPC). C152 fluorescence lifetimes were measured as a function of sample temperature, and changes in the relative contributions of these lifetimes (corrected for quantum yield) to the overall emission decay data were attributed to changes in the distribution of C152 solutes between the aqueous buffer, the polar vesicle headgroup region, and the hydrophobic interior of the vesicle bilayer. When the bilayer was in its more rigid, gel state, C152 remained predominantly in the aqueous buffer. Upon melting to its liquid crystalline state, each bilayer showed evidence of accommodating more C152 into a polar region associated with the lipid headgroups. At no temperature did C152 show strong affinity for the bilayer's hydrophobic interior. Above 50 degrees C, this behavior reversed itself with C152 moving back out of the vesicle membrane and into the buffer. All observed changes in partitioning behavior were reversible. The interesting temperature dependence of C152 partitioning suggests that C152 solvation within the lipid headgroup region represents a sensitive balance between enthalpic and entropic contributions with C152 accommodation by the bilayer being exothermic but entropically unfavorable.