performic acid ratio of 9 1 by volume the peracids are prepared by bringing the acetic and formic acids into contact with hydrogen peroxide having a concentration of 50 by weight minimum temperature 60 u00b0 c duration 3 hours the concentration of hydrogen peroxide is of the order of 0 35 mol per liter of mixture of organic acids the water content of the raw pulps is less than 10 by weight when the three hours have elapsed the pulp is filtered pressed manually and washed with distilled water the concentration of peracids in the acids recovered is 0 2 mol per liter while this concentration in the starting acids was 0 3 mol per liter this hydrogen peroxide is 0 08 mol per liter while this concentration was 0 15 mol per liter in the initial mixture of organic acids and peracids the acids recovered are enriched with hydrogen peroxide so as to serve in a new bleaching operation the pressed pulps after washing with distilled water are brought into contact with a basic solution of hydrogen peroxide containing 4 by weight of sodium hydroxide relative to the dry pulp ph of the order of 10 and 2 by weight of.