Spin-trapping E.S.R. spectroscopy has been used to demonstrate that chlorine radicals (Cl°) were formed in conditions simulating chlorine dioxide (ClO2) bleaching, in presence of iron cations. Chlorine radicals (Cl°) most probably originate from reaction of Fe2+/Fe3+ with HCLO2and with Cl2/ClOH, equally produced when ClO2reacted with a phenolic unit. Adding Fe2+Fe/3+ in a D bleaching stage of a softwood kraft pulp had a detrimental effect on both pulp viscosity and delignification. At the same time AOX and chlorate formation were increased. These effects were in accordance with the proposed mechanism for the formation of Cl° and showed that iron might be potentially harmful, if in a sufficient quantity in pulp or process water.