The structure of dimethyldioctadecylammonium-montmorillonite is studied by nitrogen adsorption, methanol adsorption, and small angle X-ray scattering. It was shown that this material is an interstratified structure. The DDA molecules are arranged at different angles to the clay layers. In addition, there are some DDA molecules adsorbed on the external surfaces of the clay layers which prevent nitrogen molecules from entering the interlayer space, resulting in a very low surface area.