The quantum yield of formation and kinetic behaviour of O2(1δg) in D2O dispersions of small unilamellar vesicles (SUVs) of dipalmitoyl phosphatidylcholine were studied by time-resolved detection of near-IR phosphorescence. At a SUV concentration of 26 nM, O2(1δg) is not quenched by the vesicles. It diffuses quickly through the lipid bilayer and a partition equilibrium of O2(1δg) between the lipid bilayer and the buffer is attained before decay occurs. In this equilibrium situation O2(1δg/sb>) is mostly located in the buffer phase, which permits the determination of absolute quantum yields for O2(1δg) production, φδ, by comparison of the luminescence in the dispersions with that in neat D2O. The maximal φδ values for the sensitizers incorporated in the SUV bilayer were 0.47 ± 0.09 for the dipyridyl complex of zinc(II) phthalocyanine (ZnPc), 0.35 ± 0.08 for porphycene, and 0.36 ± 0.08 for 2,7,12,17-tetra-H-propylporphycene. These values are equal to those in neat organic solvents but lower than those previously obtained in SUVs by using chemical trapping agents. The high degree of organization of the environment around the sensitizers does not influence their efficiency of producing O2(1δg). While no concentration dependence is observed for ZnPc (at least up to a local concentration of 20 mM in the bilayer), φδ for both porphycenes significantly decreases above a local concentration of 4 mM in the bilayer. This result is expected in view of previous observations on the concentration dependence of other photophysical parameters of the porphycenes in such microheterog-eneous media.