Amphiphilic block copolymers consisting of a fluid hydrophobic (poly(ethyl ethylene), PEE) and a hydrophilic (poly(ethylene oxide), PEO) block form monolayers at the air/water interface. With X-ray and neutron reflectivity, the density profile of PEE432-PEO484 was investigated. It was found that the polymer adsorption layer consists of a homogeneous PEE and a solubilized PEO block, which can be laterally compressed by a factor of 3. The PEE thickness increases in inverse proportion to the molecular area, and the PEO brush follows the scaling law predicted for a brush in a good solvent. However, the stretching of the two blocks roughens the PEE-PEO interface, causing a transition from a PEO monolayer adsorbed to a hydrophobic interface (0.8-1.1 nm thick) to a PEE-PEO/water interfacial layer of 3 nm thickness. This transition of the interfacial layer highlights the rich phase behavior of amphiphilic block copolymers, which resembles that of lipids and nonionic surfactants.