[en] The mechanism of quercetin, morin and rutin interaction with trivalent terbium ions is studied by the optical relaxation method. The investigation was carried out at pH of 2 and 5 and temperatures of 4, 25, 40 deg C. At these conditions quercetin, morin and rutin are quenchers of Tb (3) luminescence. Reduction of Tb (3) luminescence quenching time is shown to depend linearly on quencher concentration. The most probable mechanism of Tb (3) luminescence quenching is radiationless energy transfer from electron-excited Tb (3) ion to phenol compound molecules at complex formation