Polyethylene glycol (PEG)-conjugated lipids are commonly employed for steric stabilization of liposomes. When added in high concentrations PEG-lipids induce formation of mixed micelles, and depending on the lipid composition of the sample, these may adapt either a discoidal or a long threadlike shape. The factors governing the type of micellar aggregate formed have so far not been investigated in detail. In this study we have systematically varied the lipid composition in lipid/PEG-lipid mixtures and characterized the aggregate structure by means of cryo-transmission electron microscopy (cryo-TEM). The effects caused by adding sterols, phosphatidylethanolamines, and phospholipids with saturated acyl chains to egg phosphatidylcholine/1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine -N-[methoxy(polyethylene glycol)-2000 (EPC/DSPE-PEG(2000)) mixtures with a fixed amount (25 mol %) of DSPE-PEG(2000) was studied. Further, the aggregate structure in 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine/1,2-dimyristoyl-sn-glyc ero-3-phosphatidylethanolamine-N-[methoxy(polyethylene glycol)-2000] (DMPC/DMPE-PEG(2000)) samples above and below the gel to liquid crystalline phase transition temperature (T-C) was investigated. Our results revealed that lipid components, as well as environmental conditions, that reduce the lipid spontaneous curvature and increase the monolayer bending modulus tend to promote formation of discoidal micelles. At temperatures below the gel-to-liquid crystalline phase transition temperature reduced lipid/PEG-lipid miscibility, furthermore, likely contribute to the observed formation of discoidal rather than threadlike micelles.