The electrochemical oxidation of the natural antioxidant 2,3-dehydrosilybin (DHS) was investigated in acetonitrile. The spectral changes during two electron and two proton oxidation registered by in situ IR spectroelectrochemistry show that the electron transfer is followed by a subsequent chemical reaction with traces of water. A benzofuranone derivative (BF) is formed by ECEC (electron transfer-chemical reaction-electron transfer-chemical reaction) process at the potential of the first oxidation wave. A minor difference in the chemical structures of flavonolignans DHS and silybin, the presence of a double bond between atoms C-2 and C-3 in the DHS molecule, causes the formation of completely different oxidation products. BF was for the first time identified as the product of the oxidation of flavonolignan DHS. Its formation was proved by electroanalytical, chromatographic, and spectroelectrochemical techniques. Molecular orbital calculations support the experimental findings.