Mechanical pulps are bleached by treatment with oxygen gas and alkali in the presence of borohydrides. The combined treatment gives brightness gains of 15-20 points» comparable with those achieved using stabilized alkaline 4% hydrogen peroxide. By contrast the reaction of oxygen and alkali alone darkens high-yield pulps; and treatment with borohydride in the absence of added oxygen bleaches only partially, to give about half the brightness gained if oxygen is added. Evidence is presented that the bleaching occurs via the conversion of oxygen molecules (O 2 ) to hydroperoxide ions (HOO~) by reaction of the oxygen with ionized phenolic groups in the lignin. The hydroperoxide ions are thus generated in situ in the lignin where they bleach by reacting with coloured lignin quinone and coniferaldehyde chromophores. Without the addition of borohydride the lignin phenolic groups are irreversibly oxidized by oxygen to coloured products. The primary role of the borohydride is to regenerate the phenolic groups and maintain an oxygen -> hydroperoxide[(O 2 ) -> (HOO~)J cycle. In principle other reducing agents could be used as alternatives but in practice only sodium borohydrides and substituted borohydrides have been found to be sufficiently stable to the reaction conditions. In a secondary role, the borohydride also contributes to the bleaching by reducing radicals, organic peroxides and carbonyl compounds present in the pulps.