The interfacial behavior of surface localized, poly(ethylene glycol) (1PEG) 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 type Ci:j-EO151 (16≤i≤18, 0≤j≤2) adsorb similarly at MeSi-, PC-, and PA-coated surfaces, reaching a plateau (≈0.1 PEG chains * nm-2) independent of micelle formation. When normalized for acyl tail hydrophobicity, PEG-amphiphile adsorption is relatively non-cooperative and independent of acyl tail; saturation being largely determined by repulsive PEG inter-chain 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 ≈0.07 g * cm-3, and greater than in the PEG-enriched phase of >many two-phase systems. This suggests 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 reflected by the partition of analogous microparticles. These results help explain previous observations that changes in partition induced by surface localized PEG, mirror the effects of such localization on the physiological behavior of bio-active colloids.