We focus on the role of water in a protein-resistant poly(ethylene glycol) (PEG) layer. Using the combination of two experimental techniques, namely, the extended surface forces apparatus and the quartz crystal microbalance, we demonstrate that the water content inside these surface-grafted layers is over 80 vol % while the conformational space of the PEG chains is significantly modulated in water. Discrete and reversible film thickness transitions of 1.25 Angstrom size are shown to occur when the film is compressed, a finding that suggests a high degree of organization in the PEG/water complex. The results are discussed in terms of the excellent protein resistance properties of this type of surface.