AbstractAn oxygen-delignified eucalypt pulp (Eucalyptus grandis x Eucalyptus urophylla hybrid)was provided by a Brazilian pulp mill. It was treated with chlorite to remove lignin, or by a long acidolysis stage to remove most of the hexenuronic acid groups (HexA) followed by a chlorite treatment to remove the lignin. Three pulp samples were treated with increasing ozone charges (up to 2% on pulp) at high consistency and room temperature. The pulp containing no lignin but all of the HexA appeared to be the most affected by the ozone treatment. This indicated that the reaction of ozone with HexA generates radicals, which then react with cellulose, leading to depolymerization. It also indicated that lignin captures some of these radicals. ESR spectroscopy confirmed that hydroxyl radicals are formed when ozone reacts with maleic acid, which can be considered a HexA model compound. These findings open the way to the improvement of chlorine-free bleaching sequences containing ozone stages.