New insights in the mechanism of alkaline darkening of (manganese-containing) wood pulps and the brightness loss observed in prolonged hydrogen peroxide bleaching are presented. From the mass and energy balance of the reactions of hydrogen peroxide with manganese compounds, it can be concluded that black MnO2 contributes to pulp darkening. MnO2 is formed from Mn(OH)(2). When the hydrogen peroxide concentration drops below about 0.016 m (the number of moles per 1 kg of water), residual manganous hydroxide is oxidized further to MnO2 by dissolved oxygen. Bleaching experiments on pulps spiked with manganese show a brightness reversal when the hydrogen peroxide concentration drops to about 0.015-0.027 m, in excellent agreement with the prediction. No brightness reversal is observed for pulps containing a negligible amount of manganese. The same mechanism can explain the advantage of high-consistency bleaching, in which local hydrogen peroxide concentrations are higher and more likely to remain above the critical concentration, below which darkening occurs.