Cryo-transmission electron microscopy (cryo-TEM) was employed to investigate the aggregate structure in dilute aqueous dispersions of egg-phosphatidylcholine (EPC), oleic acid (OA), and the lysophospholipid monooleoylphosphatidylcholine (MOPC). At physiological pH and salt concentration, a relatively monodisperse population of unilamellar liposomes was detected in mixtures containing equimolar concentrations of the three components. Threadlike micelles constituted the dominant aggregate structure in samples containing high concentrations of MOPC. Excess fatty acid forced, on the other hand, the system toward structures with net negative curvature. In the absence of phospholipid, cryo-TEM revealed bilayer fragments in coexistence with threadlike micelles in mixtures containing the same molar amount of MOPC and OA. External addition of MOPC to preformed EPC liposomes gave rise to a concentration dependent evolution of intermediate structures, including open liposomes and bilayer fragments. The structural rearrangements were found to be slow and permitted visualization of a number of interesting transition structures. In addition to the structural studies, static and time-resolved fluorescence measurements were employed to determine some fundamental parameters for MOPC micelles. The results indicate a critical micelle concentration of close to 5 muM and an aggregation number of approximately 142.