The enzymic cooxidation of beta-carotene (BC) by xanthine oxidase (XO) in aqueous : solutions leads to beta-ionone (BI) and derivatives : epoxy-beta-ionone (EPBI), dihydroactinidiolide, beta-cyclocitral, pseudoionone, etc. We demonstrate, in this article, that this is due to free radical (R.) formation during the first hour of stirring and the use of aldehydes as substrate. The bleaching of BC does not occur when the common substrate of XO, xanthine, is used; this proves that the superoxide anion O-2.- alone is not active on BC. BI formation in this case is not observed.