BSA adsorption onto bimodal PEO brushes at a solid surface was measured using optical reflectometry. Bimodal brushes consist of long (N = 770) and short (N = 48) PEO chains and were prepared on PS surfaces, applying mixtures of PS29-PEO48 and PS37-PEO770 block copolymers and using the Langmuir-Blodgett technique. Pi-A isotherms of (mixtures of) the block copolymers were measured to establish the brush regime. The isotherms of PS29-PEO48 show hysteresis between compression and expansion cycles, indicating aggregation of the PS29-PEO48 upon compression. Mixtures of PS29-PEO48 and PS37-PEO770 demonstrate a similar hysteresis effect, which eventually vanishes when the ratio of PS37-PEO770 to PS29-PEO48 is increased. The adsorption of BSA was determined at brushes for which the grafting density of the long PEO chains was varied, while the total grafting density was kept constant. BSA adsorption onto monomodal PEO48 and PEO770 brushes was determined for comparison. The BSA adsorption behavior of the bimodal brushes is similar to the adsorption of BSA at PEO770 monomodal brushes. The maximum of BSA adsorption at low grafting density of PEO770 can be explained by ternary adsorption, implying an attraction between BSA and PEO. The contribution of primary adsorption to the total adsorbed amount is negligible. (c) 2005 Elsevier Inc. All rights reserved.