In this study, the adsorption of some organic acids from aqueous solutions on modified and unmodified montmorillonite clays was carried out as a function of temperature and initial concentration of the adsorbate, through adsorption isotherms and their thermodynamic parameters (S, H, and G). The results showed that the adsorption capacity of the adsorbents increased with the concentration, at constant temperature, and decreased with the increase of temperature, at constant concentration. The negative values of G and H showed that the adsorption was a spontaneous and exothermic process. In general the experimental data were well represented by the Langmuir and Freundlich isotherms models.