Various ion exchange forms of preliminary partly decationised zeolite (hydrogen forms) were obtained by indirect modification with metal salt solutions, as well as by direct treatment of natural clinoptilolite taken from Bulgarian deposits. Direct modification leads to a higher extent of samples enrichment with corresponding ion. Independently of the conditions, the alkaline and alkaline earth metal ions (especially sodium and calcium) were inserted at a greater extent, while the transitional metals-at a comparatively lower extent. The cationic forms of clinoptilolite were used for adsorption and desorption experiments. The breakthrough adsorption curves and the concentration curves at temperature-programmed desorption were obtained and compared. The breakthrough and saturation times, the adsorption capacity, the distribution coefficient, the adsorbed SO2, the portions desorbed as SO2 and SO3, respectively, as well as the not desorbed portion of SO2, were determined using these curves. It was established that a definite quantity of undesorbed SO2 has remained in the zeolite forms modified with transitional metal cations. This statement was proved not only by the comparison between the adsorbed and desorbed quantities, but also by three-cycle adsorption-desorption experiments for the Cu2+-form. The results demonstrate a decrease in the capacity for each following cycle in an extent similar to the undesorbed SO2 quantity. It was not observed a visible difference in the values of the distribution coefficients for adsorption on identical cation forms, directly or indirectly obtained. However, the breakthrough time of the samples obtained by ion exchange of the hydrogen form was longer in all cases. Definite quantities of desorbed SO3 were registered for all forms, except for the natural clinoptilolite and the samples enriched with alkaline and alkaline earth metal cations. (C) 2009 Elsevier B.V. All rights reserved.