The main purpose of this work is to study the possibilities for removal of ammonium ions from aqueous solutions by the two coal-based activated carbons (one obtained from Bulgarian lignite from the Chukurovo deposit, and the second, available as commercial product) and their oxidized modifications. The porous texture and surface chemistry of the adsorbents were characterized. Adsorption of NH(4)(+) ions was investigated using solutions with different concentrations in the range 35-280 mg l(-1) in a Static mode. Equilibrium modeling data were fitted to linear Langmuir's and Freundlich's equations and maximum adsorption capacities were calculated. (C) 2008 Elsevier Ltd. All rights reserved.