We have used neutron reflection to study, for the first time, the structure of a surfactant layer adsorbed at the hydrophobic solid/water interface. Isotopic labeling of the water and of the hydrophobic self-assembled layer of octadecyltrichlorosilane (OTS) was first used to characterize protonated and deuteriated hydrophobic layers on the silicon oxide on the (111) face of silicon. This is the first time the structure and composition of such layers has been examined under water. Some water penetration into both the silicon oxide layer and the hydrophobic layer was observed. In the former case this is attributed to roughness of the oxide layer and in the latter to imperfections in the OTS layer. The thickness of the OTS layer was found to be 24 +/- 2 Angstrom, in agreement with other measurements in air. The adsorption isotherm of the surfactant tetraethylene glycol monododecyl ether (C(12)E(4)) was measured using two independent measurements (different isotopic compositions) on the deuteriated OTS layer. The surfactant was found to reach a constant excess at the critical micelle concentration (cmc) very similar to that at the air/water interface, i.e., an area per molecule of about 50 Angstrom(2). The thickness of the surfactant layer was also found to be similar to that at the air/water interface and decreased rapidly with decreasing coverage. Estimates of the angle of tilt of the surfactant molecules from the surface normal were 53 +/- 10 degrees and 75 +/- 10 degrees at the highest and lowest coverages, respectively. Isotopic labeling of the two halves of the surfactant molecule was used to show that the molecule is partially oriented with the ethylene glycol groups pointing outward toward the aqueous solution. At the cmc the thicknesses of the two halves of the surfactant molecule were both found to be 10 +/- 2 Angstrom, to be compared with fully extended chain lengths of 16.9 (alkyl chain) and 14.2 Angstrom (ethylene glycol chain). Some penetration of the OTS layer by the surfactant was observed at the highest surfactant coverages.