The structure of monodisperse and bimodal brushes consisting of poly(ethylene oxide) (PEO) at the air/water interface is investigated with neutron reflectivity, and the results are compared with the structure predicted by the Scheutjens-Fleer self-consistent-field lattice model. The monomer density profile of a monodisperse PEO brush at low and intermediate grafting densities is block-like with an extended tail region in which the density smoothly decays to zero. At high grafting densities, however, the profile is predominantly parabolic, as predicted by analytical self-consistent-field models. Quantitative agreement is found between the experimentally measured profiles and those predicted by the Scheutjens-Fleer model. Bimodal brushes are investigated for three chain length ratios and mixing ratios at various grafting densities. It is concluded that at a given grafting density, the long chains are more extended in bimodal brushes than in monodisperse brushes at the same grafting density. This additional stretching increases with increasing length of the smaller block or increasing fraction of smaller blocks. The agreement between the Scheutjens-Fleer and measured density profiles is also good in the case of bimodal brushes.