Abstract: The main objective of this study was to compare a newly developed isotherm model (Saad isotherm model, SIM) with five conventional isotherm models (Langmuir, Freundlich, Langmuir-Freundlich, BET and Toth) for adsorption of cobalt (Co) ion on a specified adsorbent. The Saad isotherm model fitted well the experimental data from three types of activated and non-activated Saudi clays. The Langmuir isotherm model agreed well with the experimental data of Khiber clay and Bahhah clayactivated by H2O2, NaCl and H2O4. While the Frendlich model and Langmuir-Frendlich model fitted well the experimental data of natural Tabbuk clay, activated Tabbuk clay by NaCl and the natural Bahhah clay. In conclusion, the Frendlich model fitted well the experimental data of Tabbuk clay activated by H2O2 and H2O4. Besides, the BET model did not describe the experimental data well of the three types of activated clays.