The exchange of calcium and magnesium for sodium at 298 K on Wyoming bentonite was investigated in perchlorate and chloride background media maintained at pH 7.0 and at a total cation normality of 0.05/V. In the perchlorate medium, the apparent total adsorbed metal charge was independent of the exchanger composition and equal to 0.97 ± 0.06 mole kg"1 (average for Na-Ca and Na-Mg exchanges). In the chloride medium, the apparent total adsorbed metal charge increased with the amount of calcium or magnesium adsorbed, tending to a value near 1.3 mole kg"1 as the clay became saturated with the bivalent metal cation. These results were interpreted as evidence for the adsorption of CaCl+ or MgCl+ complexes in the exchange experiments conducted in the chloride medium. A detailed analysis of the exchange data led to the conclusion that a Ca-montmorillonite or Mg-montmorillonite suspension in a chloride background consists of quasicrystals with bivalent cations adsorbed on the internal surfaces and monovalent metal-chloride complexes adsorbed on the external surfaces.