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 anti-oxidant to oxidant molecule. The kinetic parameters of the reactions with various oxidizing 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.