This work studies the influence of the metals Cd(II), Cr(III), and Hg(II) on the adsorption of 1,3,6-naphthalenetrisulfonic acid (NTS) on ozonized activated carbon. The adsorption capacity W and adsorption intensity (1/n) of NTS on activated carbon is increased in the presence of Cd(II) and Cr(III), whereas the adsorption process is not affected by the presence of Hg(II). This behavior is more marked in ozone-treated activated carbon samples. The presence of the cationic species Cd(OH)(+) and Cr(OH)(2+) in the medium enhances attractive electrostatic interactions between the carbon surface and NTS. Hg(II), because it is Hg(OH)(2) as a neutral species, does not affect the adsorption of NTS on activated carbon. Study of the influence of the solution pH on NTS adsorption in the presence of the metals Cd(II), Cr(III), or Hg(II) revealed that greater NTS adsorption is produced when the charge signs of the metal species and NTS in the solution are opposite. Analysis of the amount of metal adsorbed according to the pH, in the presence of NTS, showed that the presence of NTS in the medium increases the adsorption of Cd(II) and Cr(III) but has no effect on the adsorption of Hg(II) on activated carbon. The results obtained indicate that the adsorption of NTS on activated carbon in the presence of Cd(II) and Cr(III) occurs by a cooperative mechanism, in which electrostatic interactions play a key role.