The retention and complexation of Pb2+ in the interlayer of a smectite clay(hectorite) was investigated. Based on XRD results, the Ca-Pb exchanged clays most likely exhibit random interstratification. A random alternation of one-water-layer (similar to 12.5 Angstrom) and two-water-layer (similar to 15.5 Angstrom) hydrates as well as an irrational set of basal reflections are observed. At a specified humidity, the basal spacings and IR intensity of the water-bending mode decrease as the lead content in the clays increases. The Ca-Pb exchange isotherm clearly shows increasing selectivity of the clay for Pb2+ over Ca2+ with increasing Ca2+ concentrations in the clay. The thermodynamic equilibrium constant is 5.18 and the standard free energy change is -4.08 kJ/mol at 298 K. The higher adsorption specificity for Pb2+ over Ca2+ could be partly due to the difference in ionic potentials and hydration energies between Pb2+ and Ca2+. However, in the solid retrieved from the exchange solution, an infrared spectroscopy study has shown the formation of a lead hydroxycarbonate complex. The role eventually played by this complex in enhancing the selectivity is unknown.