Neutron diffraction, in conjunction with isotopic substitution of Li-6 for Li-7 and H-2 for H-1, has been used to determine the structure of the electric double-layer region close to hydrated lithium vermiculite clay platelets. We have measured the intensities of the first 27 (OOl) Bragg reflections for samples in which the clay platelet spacing is 14.67 Angstrom. Difference analysis has then allowed us to establish the interlayer counterion and water distributions. We find that the lithium counterions are located midway between the clay platelets and form octahedral hydration complexes with six water molecules. The behavior of lithium is therefore rather different from that of the larger alkali-metal ions; sodium, potassium, and cesium. These ions prefer to bind directly to vermiculite clay surfaces, rather than fully solvate. Since only lithium-substituted vermiculites will swell macroscopically when soaked in water, we conclude that interlayer cations must detach themselves from the clay surfaces if the particles are to expand colloidally.