Spread monolayers of diblock copolymers of styrene and ethylene oxide at the air-water interface were studied by surface pressure measurements and neutron reflectivity, as a function of coverage sigma and chain length N. The surface pressure data have three regions, one at low coverage, where a relatively sharp increase due to increasing intermolecular interaction is found, a more gently increasing part at intermediate coverage, where the poly(ethylene oxide) block gradually desorbs to form a blush, and a sharply increasing part at high coverage, where the blush is compressed. The neutron reflectivity measurements, taken in the intermediate and high coverage regions, confirm the presence of a brush with a thickness scaling, by approximation as sigma(1/3). These brushes could be compressed by a factor of about 5 without desorption occurring. The observations are in good agreement with numerical calculations based on a mean field lattice model for terminally anchored, adsorbing chains. These calculations predict a gradual change in the average configuration from a flat, adsorbed state to a brush consisting of stretched chains.