Retention rate of Cr(VI) and Hg(II), from an aqueous medium by activated carbons obtained from peach stones and Eucalyptus globulus chips, is reported. To describe the retention rate of these ions, the effects of the activated carbon preparation variables: activating agent (steam and CO2), and burn-off, are analyzed. In addition, the effect of the following adsorption process variable: initial concentration of the metallic ion, system temperature, pH of the medium and particle size of the adsorbent is discussed. Cr(VI) was retained at a higher rate in E globulus- activated carbon than in peach stone-activated carbon due to the higher volume of the transport pores in the former material. The retention rate appears to include a micropore diffusional control in the Hg(II) retention and a transport pore diffusion control in Cr(VI) retention. The retention rate of Hg(II) is also dependent on chemical functionality of the activated carbon surface and the retention is enhanced when carbon dioxide is used as activating agent, whereas Cr(VI) retention is higher when steam activation is used.