This work is concerned with the role of oxygen on the selectivity of chlorine dioxide in the predelignification stage of a E. globulus pulp bleaching plant. Its main purpose is to study the selectivity of chlorine dioxide when applied to an oxygen pre-delignified hardwood kraft pulp and to compare it to that of a conventional pre-delignification with chlorine dioxide (D). The intrinsic viscosity and kappa number were used to follow the polysaccharides degradation and the delignification rates. In an oxygen stage (O), the selectivity varies with the degree of delignification. In the initial fast phase of reaction it exhibits values similar to those of chlorine dioxide in a conventional pre-delignification stage (D), although it diminishes drastically in the second, slow phase of reaction. On the other hand, chlorine dioxide in a conventional pre-delignification stage (D) is a very selective agent and its selectivity remains practically constant along the process. Moreover, it's shown that it is independent of the initial viscosity and kappa number of pulp. However, when chlorine dioxide is applied to a previously oxygen delignified pulp, its selectivity decreases gradually with the extent of delignification in the preceding oxygen stage. The experiments suggest that the global selectivity of an OD sequence with Eucalyptus globulus kraft pulp can be improved if the delignification with oxygen is limited to the fast phase of reaction, which corresponds to the first 5-10 min of reaction.