The mechanisms of Cr(III) sorption by pine bark treated chemically (MPB) with NaOH were investigated. The removal of Cr(III) was found to be strongly dependent on the initial pH. Thus, a model was developed for describing the adsorption as a function of this variable. The removal efficiencies as a function of the adsorbent dosage for different pH values were predicted. The equilibrium data were well fitted by the modified Langmuir isotherm. Ion exchange at equilibrium suggested chromium uptake and Ca2+ release occurs approximately according to the stoichiometry of the equilibrium reactions. Cr(III) desorption from MPB with chelating agents was not very effective while high efficiencies were achieved with HNO3 solutions.