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Abstract: A paradoxical relation between the secondary emission mass spectra of Methylene Blue and its leuco form is revealed: in the mass spectrum of the dye cation Cat+ (oxidized form with the molecular weight 284), an intense product of its reduction with m/z 285 is recorded, while the mass spectrum of the leuco dye (reduced form with the molecular weight 285) corresponds to an oxidized form with a dominating peak at m/z 284. An explanation of this empirical fact is proposed: the redox reactions induced by ionizing factors under the conditions of secondary emission experiments pass in the direction permitted for the particular initial form of the redox-active compound. Namely, an oxidized form undergoes reduction, whereas the reduced form is oxidized. This effect should be taken into account for the correct identification of redox-active compounds by their secondary emission mass spectra and for simulating redox reactions in the systems containing redox-active dyes under mass spectrometric conditions. [ABSTRACT FROM AUTHOR]