Abstract Small-angle X-ray scattering and transmission electron microscopy were used to determine the microstructure of Ca- and Na-smectite pastes. Samples were examined during drying and during rewetting at room temperature in an ultrafiltration cell by varying the suction pressure. The microstructure of Ca-smectite in (10− 3 M CaCl 2 ) at suction pressures corresponding to high water contents ( ≤ 1 bar) appears to consist of a network of quasi-crystals containing pores about 1 μ m in size. The network walls result from the face-to-face bonding of 50 to about 400 layers, depending on the suction pressure and the previous degree of drying. For Na-smectites in a 10 −3 M NaCl solution at low suction pressure ( ≤ 0.1 bar), the tactoids were found to consist of a few layers (≤10), and the water was found to be located essentially between the layers inside the tactoids.