Co2+ and Zn2+ ions are adsorbed on cryptomelane-type MnO2 by exchange with surface protons and with structural ions (probably K+ and/or Mn2+) in the oxide. The latter sites are responsible for the much higher capacity to these cations, compared to Na+. At all pH values, two straight lines expressing the presence of mainly two groups of sites with distinctly different adsorption energies are located in the Langmuir plots for both Co2+ and Zn2+. The apparent capacities of the two groups increase with the increase of pH, indicating the involvement of protons in the adsorption process over the whole concentration range. The higher Co2+ capacity at relatively low pH, compared to the Zn2+ capacity, is probably due to a more exchange with the structural ions. Crytomelane type MnO2 seems to be a quite heterogenous ion adsorbent whose adsorption sites could be approximated to two groups only.