Abstract One undesirable reaction during ECF bleaching is the formation of resistant quinones during the oxidation of lignin phenolic rings by ClO2. Occurrence of this side reaction was confirmed by the implementation of a new in situ method for quinone detection. It is based on the selective reaction of free-phenol groups in pulp with ClO2 at low temperature (0°C) and on the reduction of quinones into free phenols by sodium dithionite. Although not absolutely quantitative, it gives valuable information on the quinone content without any need of lignin extraction. It is proposed to apply a hydrogen peroxide treatment after each ClO2 (D) stage to solve this problem. Another undesirable reaction is the consumption of ClO2 by the muconic acid type structures which are the primary oxidation products of lignin during a D stage. Experiments with Z,Z-2, 5-dimethyl muconic acid, a model for lignin oxidized units, indicated that this reaction is most probable. Although phenolic moieties in lignin are more reactive than the muconic acid structures, the use of high ClO2 charges respective to lignin may favour its consumption by the muconic acid structures. One solution to this problem is to apply lower charges of ClO2 in each D treatment. The relevance of these findings was demonstrated by the high efficiency of (DEP)(DEP) sequences, where DEP stands for a ClO2 stage at low charge of chlorine dioxide immediately followed by an alkaline hydrogen peroxide treatment.