Abstract: Montmorillonites saturated with Li+, Na+, K+, NH4+, Mg2+, Ca2+, Ba2+, or Ni2+ ions can form complexes with propylene carbonate (PC) by intercalation; the d(001) of montmorillonite expands to 19 Å. In the infrared absorption spectra of these complexes, the C=O stretching band of the intercalated PC molecules shifts to lower frequencies, and the amount of the shift increased with the increase of the polarizing power of the interlayer cations. Water molecules were strongly bound to the cations. The PC molecules interacted with the cations by way of H2O molecules. With the removal of H2O, the PC molecules directly coordinate to the cations and the PC molecules show a much larger red-shift in the C=O frequency. The PC-montmorillonite complexes exhibited osmotic swelling, even in aqueous electrolyte solutions. This finding is interpreted in terms of the formation of thick electric double-layers consisting of PC and H2O between the 2:1 layers.