A novel surface force apparatus has been utilized in order to determine the forces acting between two glass surfaces rendered hydrophobic by silanation. It was found that in pure water the long-range force is due to a repulsive double-layer force. At shorter separations the force is attractive and stronger than expected for a van der Waals attraction. When the pH is lowered to 2, the repulsive double-layer force is insignificant and the measured force is attractive at all separations. The measurable range is about 30 nm. The attractive force decays roughly exponentially with surface separation having a decay length of 5.6 nm, significantly larger than the electrostatic Debye length (3 nm). Thus we conclude that the origin of this attractive force is not electrostatic. The forces acting between one hydrophobic silanated glass surface and one untreated glass surface at distances greater than 3 nm are dominated by a repulsive double-layer force. At shorter separation the force turns attractive. The attractive force observed is slightly larger than that expected for a van der Waals force but significantly smaller than that observed between two hydrophobic surfaces or between mica and hydrophobized mica.