We report measurements of the interaction force between functionalized probe tips and oligo(ethylene glycol) (OEG)-terminated monolayers. Force profiles obtained with the interfacial force microscope indicate the presence of an interphase water layer adjacent to the OEG-terminated monolayer that generates a repulsive hydration force. This interphase layer extends up to 5 nm from the surface and has a viscosity that is 6 orders of magnitude greater than that of bulk water. The possible role this interphase layer plays in the exceptional protein resistance of such systems is discussed.