The effects of poly(ethylene glycol) (PEG) on sonicated unilamellar vesicles made of phosphatidylcholine have been examined. Stopped-flow and equilibrium data are presented for vesicle aggregation, vesicle leakage, lipid mixing, and aqueous contents mixing. Vesicle aggregation is detected as a monoexponential increase in light scattering, for PEG concentrations between 5 and 10%. In the region between 10-15% and 23-27% PEG, under our experimental conditions, the increase in light scattering follows a more complex biexponential kinetics, and, under these conditions, vesicle aggregation is accompanied by lipid mixing, a combination of events denoted as "close apposition". Above 23-27% PEG, the increase in light scattering is accompanied by fast lipid mixing, and also mixing of aqueous contents, all this being indicative of vesicle fusion; in addition, leakage occurs under these circumstances. Fusion takes place at high PEG concentrations, as indicated above, without any dilution step. From a methodological point of view, the Tb/DPA assay is shown to be more appropriate than the ANTS/DPX method for leakage and fusion studies in the presence of PEG.