Over the last few years, ozone has been considered as an effective reagent for the bleaching of chemical pulp and an alternative to conventional chlorinated compounds which induce environmental pollution. The purpose of this work was to investigate the optimal conditions of the ozonation process with the aim of avoiding the breakdown of carbohydrates present in the pulp. In order to limit the depolymerization of cellulose, which involves a decrease in the pulp strength, various organic compounds called "protectors" were added to the reaction medium. The results reported in this paper show the important role of most additives and their influence on the hydrodynamic phenomena taking place in the ozonation process. Indeed, these products improve the mass transfer and, more particularly, the diffusion of ozone in the pulp. Consequently, their presence enhances the performance of the ozonation stage.