Interactions between hexadecyltrimethylammonium bromide (HTAB) and L-α-phosphatidylcholine (PC), 1,2-didecanoyl-sn-glycero-3-phosphocholine (DPC), and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (PPC) at the air/water interface and in the bulk solution were evaluated using interfacial tension, fluorescence, and conductivity measurements to study the aggregation processes of HTAB in fixed concentrations of aqueous lipid solutions containing 7-36 µM of each lipid. The interfacial and fluorescence measurements showed the occurrence of two kinds of aggregation processes in HTAB-PC and HTAB-DPC systems, which were identified by the three breaks at low lipid concentrations, i.e., 7-14 µM. The first aggregation process, C 1 , involved the incorporation of HTAB monomers into the lipid vesicles and the subsequent disruption of the vesicular structure leading to the formation of mixed micelles. The second aggregation process, C 2 , which started at the second break, indicated the completion of the mixed micelles and the initiation of independent micelle formation process which was completed at the third break. At higher PC or DPC concentration, i.e., 14-22 µM, the first break, which indicated the initiation of the first aggregation process, was not observed; only the second and third breaks were visible. This is ascribed to the presence of comparatively large vesicles, which were able to accommodate HTAB monomers initially without any drastic change in their architecture. In the presence of PPC, the aggregation process of HTAB was similar to that in pure water as studied from the interfacial tension measurements, whereas the bulk behavior indicated the presence of strong structure transitions upon incorporation of HTAB monomers in the PPC vesicles. A difference between the aggregation behaviors of HTAB with PPC was explained on the basis of the stronger hydrophobicity of PPC over PC and DPC.Paper no. S1255 in JSD 5, 123-130 (April 2002).