A new mechanism is proposed for the heterogeneous alkaline peroxide brightening reactions of mechanical pulps. The mechanism consists of four key kinetic steps: adsorption of hydrogen peroxide and hydroxide to the pulp fiber walls; chromophore-removing chemical reaction on the fiber wall; desorption of ''light'' organic products formed from the fiber wall; and oxidation chain reduction of the cleaved organic substances. The most important step here is the surface reaction, rather than reactions occurring in the liquid phase. In general, the removal of the cleaved organic substances from the fiber wall is not anticipated to occur completely during the brightening reaction operation stage. The failure of the "light" organic products to completely dissociate from the fiber wall is more pronounced at high consistencies and when agitation is weak. The pulp brightness is measured after washing the pulp and thus removing the "light" organic products. The overall brightness enhancement or chromophore-removal rate is thus limited by the reaction occurring on the fiber wall. A kinetic model has thus been developed based on the proposed mechanism. An apparent second-order reaction in terms of the chromophore concentration, first orders in terms of the adsorbed hydrogen peroxide and adsorbed hydroxyl concentrations are observed from the kinetic model. The derived kinetic model can explain the existing experimental data from various sources quite well. (C) 2003 Elsevier Science Ltd. All rights reserved.