Neutron reflectometry is used to investigate the structure of polymers end-grafted to an interface at high grafting densities, so-called brushes. Our system consists of polystyrene-polyethyleneoxide (PS-PEO) diblock copolymers, irreversibly adsorbed at the air/D2O interface. At relatively low grafting densities the density profile of a monodisperse brush is block-like, with a tail region in which the density smoothly decays towards zero. At high grafting density the profile is predominantly parabolic. Bimodal brushes are examined for several length ratio's and mixing ratio's. At a given grafting density the extension of the long chains is larger in a bimodal brush than in a monodisperse. This additional extension increases with increasing length and fraction of small chains. Good agreement is found between the density profiles obtained from analysis of the neutron reflectometry data and profiles predicted by the Scheutjens–Fleer self-consistent-field model.