The influence of polyphenol oxidase and ascorbate oxidase on radical-scavenging activity and contents of total phenol, chlorogenic acid, and ascorbic acid in vegetables during the cooking process were investigated. In the case of burdock and lettuce, which have a high activity of polyphenol oxidase, the radical-scavenging activity and the content of total phenol and chlorogenic acid decreased drastically within 1 min. In the case of broccoli, however, only a small decrease of radical-scavenging activity was observed, and total phenol and chlorogenic acid decreased almost not at all. The decrease of the activity in broccoli depended on the oxidation of ascorbic acid by ascorbate oxidase. None of these compounds decreased after the enzymes had been inactivated by heating.