To avoid any uncertainty, a specially synthesized deuterium labeled all-trans 2,4-pentadienol (X= -OH) and its acetate (X= -OAc) were taken as substrate RX for combined direct / HOMEC electrolysis . The feature of the substrates is that they produce the same protonating pentadienyl anion (PDA), a symmetric anion, with equal maximal electron densities at the terminal atoms C 1 and C 5 . Therefore, the electrolysis products, deuterated pentadien (piperylen) isomers A and B, should be obtained in strictly equal amounts (scheme 3). In fact, as was shown by 1 H and 2 H NMR, electrolysis leads to the predominant formation of one isomer A >>B; this was not the cas e f or the НОМЕС electrolysis. These data were confirmed by the results of a more detailed study including also RX methyl ether (Table , Fig. ); description of NMR-spectra are in , the original NMR spectra and the content of cis-, trans-isomers, separately for A and B, are in (see Supplement).