The adsorption of poly(ethylene glycol) (PEG)-esterified fatty acids at methylated silica, phosphatidic acid, and phosphatidylcholine surfaces was investigated with in situ ellipsometry. For a series of PEG-fatty acid esters of ethoxy groups and acyl tails of type C-i:j-EO(151) (16 less than or equal to i less than or equal to 18, 0 less than or equal to j less than or equal to 2)adsorption at methylated silica was independent of bulk micellization, and a plateau was reached below the critical micellization concentration (CMC). The plateau adsorbed amount for the investigated fatty acid esters was only weakly dependent on the nature of the hydrophobic moiety. Instead, saturation adsorption was largely determined by the interactions between PEG chains. Adsorption isotherms were therefore essentially identical on all three of the quite different surfaces. At saturation adsorption, the adsorbed layer thickness was 10-15 nm, while the average adsorbed layer concentration was 0.07 g/cm(3). Formation of the PEG-surfactant coatings thus appeared to involve significant molecular alterations of PEG from a random coil, The ability of the PEG-ester coatings to inhibit protein adsorption was also investigated. At the adsorption plateau, all coatings investigated displayed quite good ability to inhibit adsorption by a number of serum proteins. For the surfaces studied this ability decreased below 0.2 CMC. These findings are discussed in relation to the ability of PEG-derivatized lipids to control the in vivo fate of colloidal drug carriers.