In the oxidation of carotenoids, ethyl all-trans-8'-apo-beta-caroten-8'-oate and all- trans-beta-carotene, with ferric chloride, several equilibria occur between Fe3+, Fe2+, Cl-, the neutral carotenoid, and its radical cation and dication. The radical cation and dication were found to abstract an electron from Fe2+. Isomerization of carotenoids occurs during the oxidation. In the presence of air, a stable product is formed in high yield during the oxidation. H-1 NMR, LC-MS, and optical studies show that this product is the 5,8-peroxide of the starting material. A mechanism for the formation of this compound is proposed.