We have measured the force acting on neutral tips as function of distance to hydrophobic surfaces in aqueous solutions. The unusually large magnitude of this force is attributed to an electrostatic response of the aqueous fluid structure (hydration layer). The exchange of a volume of this region with a dielectric permittivity epsilon(int) by the tip with a dielectric constant epsilon(tip) is responsible for the tip attraction when it is immersed in the hydration layer. Hydrophobic hydration layers, characterized by a variable dielectric permittivity profile, have measured widths of similar to4 and similar to8 nm for hydrophobic silicon and CTAB monolayer covering mica surfaces, respectively. (C) 2003 Elsevier Science B.V. All rights reserved.37541795540546