Abstract: The aim of this study is to investigate the adsorption of cobalt ions on three types of natural Saudi clays. These three natural clays were Tabbuk clay, Khiber clay and Bahaah clay. The effect of pH on the adsorption of cobalt ions was studied. The maximum adsorption capacities of cobalt ions on the three natural Saudi clays increased with increasing the temperature of cobalt solution. The maximum adsorption capacity at 20°C was 4.66 mg g-1 for Khiber clay, 3.94 mg g-1 for Tabbuk clay and 3.44 mg g-1 for Bahhah clay. Whereas, the maximum adsorption capacity of cobalt ions on the three types of natural Saudi clay at 80°C was 5.42 mg g-1 for Khiber clay, 4.22 mg g-1 for Tabbuk clay and 3.82 mg g-1 for Bahhah clay. The enthalpy, Gibbs free energy and entropy for the adsorption of cobalt ions on the three types of natural clays were calculated. It was observed that the adsorption process of cobalt ions on three types of natural Saudi clays was an endothermic process, thus increasing the temperature increased cobalt ions adsorption on all types of natural clays. This indicated that the adsorption process is preferred at high temperatures. The equilibrium adsorption data were correlated using Langmuir, Freundlich and Redlich-Peterson equilibrium models. The experimental data of cobalt ions adsorption on the three types of natural Saudi clays correlated well with Freundlich model.