Mixtures of poly(tert-butyl methacrylate) (PtBMA) and the lipids monoolein, diolein, and triolein spread as monolayers at the air−water interface have been used as models to examine the surface phase behavior of lipid−polymer mixtures as a function of lipid structure, polymer molecular weight (17K to 3400K), and mixture composition. A measure of miscibility and polymer−lipid interactions was obtained through analysis of surface pressure−area isotherms and by direct observation with fluorescence microscopy. At all molecular weights of PtBMA, for the polymer alone and in mixtures, identical surface pressure−area isotherms were obtained when area was expressed on a per monomer basis, indicating that all monomer segments were located at the interface and equally accessible to the lipid molecules. Through thermodynamic analysis, it was found that monoolein, with two hydroxyl groups, formed nonideal miscible mixed monolayers with PtBMA under what appear to be good solvent conditions. Diolein, with one hydroxyl group, behaved very much as a ϑ solvent, forming nearly ideal homogeneous monolayers, while a lack of affinity between triolein and PtBMA due to poor hydrogen bonding led to surface phase separation at higher surface pressures, consistent with poor solvent conditions.