Butylammonium vermiculite clay gels with c-axis spacings in the range 100-300 Angstrom were studied by neutron diffraction in the Q-range 0.2-2.0 Angstrom(-1), permitting a determination of the interlayer structure in the intermediate range between 5 and 30 Angstrom. Differences between scattering patterns obtained from gels prepared with protonated and deuterated butyl chains showed that the clay surfaces are covered by a 6 Angstrom thick laver of water molecules, corresponding to two layers. The results partly validate the idea that clay swelling involves the formation of partially ordered water layers on the clay surfaces. The butylammonium chains sit between 11 and 14 Angstrom from the center of the clay platelets in a 3 Angstrom thick layer. Continuum electrical theories of clay swelling can thus only be valid at distances greater than the 28 Angstrom thickness of the clay platelet plus adsorbed layers of water molecules and counterions. The possibility of using isotopic substitution in neutron diffraction experiments to obtain quantitative information about ion adsorption at colloidal surfaces is discussed.