The interfacial behavior of surface localized, poly(ethylene glycol) (PEG) esterified fatty acid amphiphiles was comparatively studied at microparticle surfaces via phase partition and at flat surfaces via in situ null ellipsometry. Ellipsometry was performed on methylsilane (MeSi), phosphatidic acid (PA), and phosphatidylcholine (PC) coated silica slides, while human erythrocyte and PC liposomes were subjected to partition in aqueous PEG, dextran two-phase systems. Analogous results from both methods suggest that PEG-amphiphile adsorption can be relatively independent of the underlying surface. Ellipsometry indicated that members of a series of PEG-fatty acid esters of the type Ci:jEO151 (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) adsorb similarly at MeSi-, PC-, and PA-coated surfaces, reaching a plateau (approximate to 0.1 PEG chains.nm(-2)) independent of micelle formation. When normalized for acyl tail hydrophobicity, PEG-amphiphile adsorption is relatively noncooperative and independent of the acyl tail; saturation is largely determined by repulsive PEG interchain interactions. At saturation, EO151-ester layers were 10-15 nm thick, suggesting close packed PEG molecules unfolded normal to the target surface. Ellipsometry also indicated the average PEG concentration in the layer was approximate to 0.07 g.cm(-3), and greater than in the PEG-enriched phase of many two-phase systems. This suggests that the partition of PEG-coated colloids reflects interfacial free energy differences between solution- and surface-localized, polymer-enriched phases. PEG-ester adsorption (mg.m(-2)) isotherms determined on flat slides via ellipsometry correlate directly with those physiological behavior of bioactive colloids. These results help explain previous observations PEG mirror the effects of such localization on the physiological behavior of bioactive colloids.