The smectite group of clay minerals (smectites) consists of negatively charged clay layers and interlayer exchangeable cations. They are spontaneously delaminated in water to form single clay layers when the interlayer cations are small alkaline cations such as Na+ or Li+. This phenomenon known as osmotic swelling has fundamental importance in constructing novel clay-based nanomaterials. However, osmotic swelling of smectites has not been systematically investigated in organic solvents although this phenomenon should be useful for developing novel clay-organic nanocomposites. We report herein that montmorillonite, a typical smectite, with monovalent and divalent inorganic interlayer cations shows osmotic swelling accompanied by delamination of clay layers in water-acetonitrile and water-2-propanol mixed solvents, although inorganic interlayer cations have been believed to be inappropriate for delamination of smectites in organic solvents. The delamination is confirmed by a combination of macroscopic sample appearances, XRD patterns, and SEM images. Montmorillonite with interlayer Na+ or Li+ ions shows osmotic swelling in pure water and the mixed solvents but not in pure organic solvents. Montmorillonite with alkaline earth dications in the interlayer spaces is swollen in water-organic mixed solvents but not in either pure water or organic solvents alone. Partial delamination in several systems can be clarified from SEM images even though the sample appearances and XRD patterns do not give firm evidence. Such non-uniform swelling behavior of montmorillonite is related to the disordered stacking of the aluminosilicate layers with different morphologies in the clay powders as observed by SEM.