Abstract: Adsorption of Ni(II) by Ca- and Na-saturated kaolinites was studied in equilibrating solutions with total Ni concentrations ranging from 118 to 946 µ/liter. Background electrolytes used in these experiments were 0.005, 0.01, 0.025, and 0.5 M Ca(NO3)2, 0.002 and 0.005 M CaSO4, 0.01 and 0.1 M NaNO3, and 0.005 and 0.05 M Na2SO4. Ion speciation in equilibrium solutions was calculated by the computer program GEOCHEM. Computed Ni2+ concentrations and activities at equilibrium were correlated with total Ni adsorbed by kaolinite. Increasing ionic strength resulted in decreasing Ni adsorption. Adsorption of Ni was greater from solutions when NO3 was the dominant anion. Based on adsorption data in SO4 medium, the standard free energy of adsorption of Ni2+ ion on kaolinite was computed to be −27 kJ/mole.