Two common clay minerals, kaolinite and montmorillonite, are activated by treating with 0.25 M H2SO4, and the effects of acid treatment are monitored with XRD and FTIR measurements and by measuring changes in surface area and the cation exchange capacity (CEC). Kaolinite, montmorillonite, and their acid-activated forms are used as adsorbents for removing Cd(II) from water in a batch process with variations in initial Cd(II) concentration, amount of clay, pH, time, and temperature. Different models of kinetics and isotherms are tested by the data-fitting procedure. Kaolinite is much inferior to montmorillonite with respect to Cd(II) adsorption, and acid activation enhances adsorption to different extents for the two clays. Langmuir monolayer capacity has the values of 9.9, 11.4, 32.7, and 33.2 mg g(-1) for kaolinite, acid-activated kaolinite, montmorillonite, and acid-activated montmorillonite respectively. Adsorption follows an endothermic path with decrease in entropy and Gibbs energy.