ABSTRACT In the interaction between grapevines and Botrytis cinerea, one of the main aspects of pathogenicity is fungal ability to degrade phytoalexins synthesized by the plant in response to infection. Laccase-like stilbene oxidase activity in liquid cultures of B. cinerea has been shown to be related to the decrease of phytoalexin concentrations. Recent research and results presented in this paper determined the chemical structure of a pterostilbene metabolite produced by B. cinerea. Study of degradation of pterostilbene that has just one free hydroxy phenyl group function allowed us to determine the oxidative dimerization process undergone by grapevine phytoalexins after B. cinerea infection. The phytopathological significance of this degradation process in the B. cinerea interaction has also been discussed.