Recent surface theology and film balance experiments on monolayers of PEG lipopolymers at the air-water interface showed that the PEG chains are able to form a quasi-two-dimensional physical polymer network if forced into a highly stretched brushlike configuration. To obtain a deeper understanding of the complex film balance and theological transition behavior of lipopolymers, we performed surface theology and film balance experiments on phospholipid (DMPC: 1,2-dimyristoyl-sn-glycero-3-phosphati-dylcholine)/PEG lipopolymer (DSPE-PEG2000: 1;2-distearoyl-sn-glycero-3-phosphoethanolamine [poly-ethylene glycol) 2000]) mixtures at the air-water interface. We found that the high-film-pressure transition observed between 40 and 100 mol % lipopolymer at about 20 mN/m, which is related to a first-order-like alkyl chain condensation, is a necessary requirement for the existence of a theological transition. While the theological transition appeared at a specific area per lipopolymer of 165 Angstrom (2), thereby being independent of the amount of phospholipids incorporated, the area per lipopolymer at the high-film-pressure transition clearly depends on the lipopolymer-phospholipid molar concentration. Our data clearly support the Flory model of physical gelation, which predicts no thermodynamic transition at the gel point, because the isothermal compressibility and its derivative show no discontinuity at this paint. The pi -A isothermal behavior at the high-film-pressure transition of the phospholipid/lipopolymer mixtures can be interpreted if we assume that microphase separation occurs between phospholipids and lipopolymers. Our data indicate that the two-dimensional physical network of lipopolymers is formed by two different kinds of associative interactions: (1) microcondensation of alkyl chains of lipopolymers to small clusters; (2) water molecule mediation of the interaction between adjacent PEG clusters via hydrogen bonding.