Although amphiphilic polystyrene-poly(ethylene oxide) diblock copolymers (PS-PEO) adsorb from aqueous solutions to hydrophobic surfaces, they are unable to form an end-anchored brush. Four independent techniques-scanning angle reflectometry, hydrodynamic layer thickness measurements, streaming potential measurements, and total internal reflection microscopy-indicate that PS-PEO adsorption is indistinguishable from PEO homopolymer adsorption. The surface concentrations attained by three PS-PEO diblocks are independent of molecular weight (between 67 000 and 479 000) and are indistinguishable from PEO (420 000 molecular weight) surface concentrations. Hydrodynamic layer thicknesses and electrokinetic layer thicknesses are on the order of only a few nanometers for both PEO and PS-PEO diblocks, more characteristic of a homopolymer layer than an extended brush. Furthermore, adsorbed PS-PEO imparts no detectable steric repulsion to the energy of interaction between a Brownian particle and a wall. Apparently, brush formation is prevented because the surface affinity of the large water-soluble PEO block presents a large kinetic barrier to its being completely displaced from the surface by the insoluble PS block.