Mangostanate, a new prenylated xanthone, 1,3,6-trihydroxy-2-(3-methylbut-2-enyl)-8-(3-formyloxy-3-methylbutyl)-xanthone, has been isolated from the pericarp of Garcinia mangostana, together with five known compounds. The structures were elucidated using spectroscopy. All the components were tested for antioxidant activity.