Abstract: A slurry of cellulosic fiber and water having a consistency in the range of 0.017-4.9% of the total ungassed weight of fiber and water is bleached with ozone. This is done without the usual addition of organic additives. The water can include impurities created by the bleaching process. Rapid reaction times under 8 minutes and preferably under 5 minutes are achieved.The reaction is enhanced in the consistency range of about 0.017-0.7%.The slurry is mixed using a mixing energy of 0.002-1.0 horsepower per cubic foot of gassed slurry. The mixing energy will determine whether the gas-liquid or liquid-solid interface will limit the speed of the reactions. The passage of ozone from the gas to a liquid phase will be the limiting factor below about 0.2 horsepower per cubic foot of gassed slurry. The increasing presence of ozone in the liquid as the horsepower increases from about 0.2 to about 0.