Surface pressure isotherms and ellipsometric measurements of monolayers of two triblock symmetric copolymers, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), at the air-water interface have been carried out. These copolymers are water-soluble, and the difference in hydrophobicity between the blocks is small. This represents a different scenario for brush formation than for most of the hydrophobic--hydrophilic block copolymers reported so far. The surface pressure curves show two different phase transitions. The ellipsometric measurements indicate a thickness transition when the monolayer saturates, which supports the hypothesis for brush formation. The experimental data have been analyzed in terms of the scaling theory of adsorption of polymer brushes. Despite the possibility of diffusion from the interface, the PPO block acts as an efficient anchoring element in the formation of an adsorbed brush, once the adsorption sites at the interface are fully occupied. This is analogous to what has been reported for diblock copolymers with a much larger difference in the hydrophobicity of the blocks.