One of the most efficient methods of desulfurization of flue gases involves the reaction of SO2 with activated soda in a dry system. In this work, reactivities of activated soda produced from trona by direct calcination and by spray drying of trona solution were compared and a reaction mechanism was proposed. Due to the textural variations of the solid during the reaction and due to the increased significance of adsorption of SO2, at low temperatures, a two step reaction behavior was observed. Predictions from the deactivation model gave satisfactory agreement with the experimental data.