Homoionic montmorillonites saturated with several kinds of cations can form complexes with propylene carbonate (PC) by intercalation. The PC molecules interact with the interlayer cations by way of H2O molecules between the silicate layers. The PC-montmorillonite complexes exhibit osmotic swelling even in aqueous electrolyte solutions. This is interpreted in terms of the formation of thick electric double layers consisting of PC and H2O between the silicate layers. Although the PC molecules are gradually hydrolyzed to propylene glycol between the layers, the hydrolyzed complex also shows a similar swelling power in aqueous electrolyte solutions.