Antioxidant properties of synthetic 2,5-substituted 6H-1,3,4-thiadiazines were comprehensively studied using potentiometric methods based on the reactions of electron transfer (ET mechanism) and hydrogen atom transfer (HAT mechanism) from the antioxidant to oxidant molecule. The kinetic parameters of the reactions with various oxid izing agents were estimated taking into account peculiarities of the transformation of 2,5-substituted 6H-1,3,4-thiadiazines in solution. The relationships "structure-antioxidant properties" are established. The role of the electron-donating and electron-withdrawing properties of substituents in positions 2 and 5 of the 1,3,4-thiadiazine ring in the manifestation of antioxidant properties is examined.