Dimeric products of the oxidation of flavonoids (including most abundant of them – quercetin) are observed rather often even in the dilute aqueous solutions. For explaining the formation of such products the free-radical and electrochemical oxidation of simpler substrate – 4-isopropyl phenol – is considered. It is shown that the products of these processes correspond with the formation of quinone methide intermediate. The formation of similar intermediates seems to be possible at the oxidation of natural flavonoids. The principal chemical property of quinone methides is conjugated addition of various nucleophylic reagents, including water, as well as the initial phenolic compounds. Just this reaction allows explaining the formation of dimeric oxidation products of both 4-isopropyl phenol and flavonoids.For identification of 4-isopropyl phenol oxidation products the technique of HPLC-MS-MS (mass numbers of most abundant signals of mass spectra) combined with evaluations of hydrophobicity factors and ground state energies of supposed reaction products were used.