The structures and phase transitions occurring during the compression of polystyrene−poly(ethylene oxide) (PS−PEO) diblock copolymer monolayers at the air−water interface have been studied by surface pressure isotherms, neutron reflectivity, and Brewster angle microscopy. At low coverage, the EO monomers adsorb at the air−water interface, but the PEO layer is not purely bidimensional. At high coverage, the EO−interface interaction becomes repulsive. A brush structure has been observed with differences between the long and the short PEO chains attributed to the difference in the accessible range of surface concentration. For the longest chain, the PEO concentration profiles exhibit a superposition of a depletion layer near the interface and discontinuous profile characteristics of a two-phase brush described by the “n-cluster” theory. For the shortest chains, the profiles are “pseudoparabolic”. Finally, Brewster angle microscopy shows that, for the long chains, the transition between the adsorbed and the brush structures is first order.