AbstractThe content of o‐diphenols and catechol oxidase activity was followed in developing apples from fruit set to harvest time. Afterwards their level drops, apparently due to conversion to other compounds and cessation of synthesis. Catechol oxidase activity shows a peak after that occurring in content of o‐diphenols. As the fruit ripens, activity drops sharply in the particulate fractions. In the ripe apple a slight amount of soluble enzyme appears. These findings were correlated with the rate of browning of apple slices and the location of enzymes and their substrate in the fruit.Browning of slices could be prevented by dipping them into solutions of I mM‐2,3‐naphthalenediol or 2·5% N‐vinyl‐2‐pyrrolidone. Addition of 1 mM‐sodium ascorbate to the dipping medium improved the efficiency of prevention of browning.