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The behauiours of clay minerals and the extent of clay-heavy metals interactions are functions of CEC and the exposed SSA of the clay minerals. The type and the quantity of double layer cations have an essential impact on this process. This work aimed to investigate the behaviour of Na-montmorillonite and Ca-montmorillonite in interaction with Pb as a heavy metal ion. The results showed that although the buffering capacity pattern of montmorillonite and different concentrations of NaCl is relatively similar, the contaminant retention property of montmorillonite is a function of the quantity of sodium ions with which the soil is treated. In Ca-montmorillonite, in spite of very small change in its buffering capacity in comparison with that of montmorillonite, a noticeable change in contaminant retention was observed. This emphasizes the limitation of buffering capacity experiment to determine the extent of soil-contaminant interaction. On the other hand, retention experiment was able to differentiate between the behaviour of these two samples. Finally, the results of the interaction process of sodium- and calcium- montmorillonite with heavy metal contaminant were compared using the XRD experiments.