The terpene lactones ginkgolide A, ginkgolide B, ginkgolide C, ginkgolide J and bilobalide, which are components of a standardized extract (EGb 761) from leaves of Ginkgo biloba, as well as ginkgolide M from roots of G. biloba were studied regarding their reaction against superoxide (O2-) and hydroperoxyl radicals (HO2) in dimethyl sulfoxide as an aprotic solvent. It was found that the ginkgolides B, C, J, M as well as bilobalide react with superoxide and its protonated form as demonstrated by EPR and UV/VIS spectroscopy. The initial reaction rate with these oxygen-derived radicals is in the order of 100 M-1/s and below. Ginkgolide A does not react with superoxide under these conditions. From these findings it can be suggested that the superoxide scavenging effect of the ginkgolides B, C, J, M and bilobalide contributes to the antioxidant properties of G. biloba.