Hysteresis in crystalline swelling of smectites was investigated with respect to the surface properties of five reference smectites, First, the samples were Na saturated and equilibrated with either 3 M NaCl or distilled water to establish initial basal spacings of 15.5 and >40 Angstrom, respectively, Then the samples were equilibrated with NaCl solutions ranging from 0.35 to 3 M, to establish osmotic control of crystalline swelling. Basal spacings of the equilibrated clays were measured by X ray diffraction, and hysteresis was quantified as the difference in weighted-average H2O activities in the equilibration solutions for the adsorption (15.5 to 19 Angstrom) and desorption (l9 to 15.5 Angstrom) transitions, The magnitude of hysteresis was similar for all five smectites and was not significantly correlated with surface charge density, percentage of tetrahedral charge, or surface area, The cause of hysteresis in crystalline swelling is attributed to both intrinsic and extrinsic processes. Intrinsic hysteresis is an inherent consequence of changes in the expansion and attraction energy levels during swelling and is believed to be caused by rigidity of the clay-water system, Extrinsic hysteresis is caused by factors that contribute additional rigidity to smectite quasicrystals, primarily thick quasicrystals. Sample history influences the magnitude of the extrinsic hysteresis.