The adsorption characteristics of cadmium ions from aqueous solutions onto two organo-ceramic adsorbents synthesized in our laboratories were studied. The main species participating in the adsorption process, determined from the chemical equilibrium, is Cd2+. Formation of a bidentate cadmium-thiol complex is proposed to explain the experimental isotherms. The kinetics of adsorption onto the sol-gel-synthesized adsorbent was studied in a differential batch reactor with recycle. Two kinetic models were tested with experimental data: chemical reaction and film/pore mass-transfer resistances. The results indicate that either model can be used to explain the adsorption process as observed from the AARD values and from the F-test. The values of fitted parameters are k(1) = 0.0021 L.g/mmol(2).min, D-p = 1.4 x 10(-7) cm(2)/s, and tau = 5.0. The last two parameters can be used to satisfactorily predict the breakthrough curve in a fixed bed by the film/pore diffusion model. A sensitivity analysis shows that pore diffusion resistances control the adsorption process.