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The temperature dependences of the equilibrium constants of two chain reversible reac tions in quinonediimine (quinonemonoimine)-2,5 dichlorohydroquinone systems in chlo robenzene were studied. The enthalpy of equilibrium of the reversible reaction of quinonediimine with 4 hydroxydiphenylamine was estimated from these data (∆H = -14.4±1.6 kJ mol -1 ) and a more accurate value of the N-H bond dissociation energy in the 4 anilinodiphenylaminyl radical was determined (D NH = 278.6±3.0 kJ mol -1 ). A chain mechanism was proposed for the reaction between quinonediimine and 2,5 dichlorohydroquinone, and the chain length was estimated (ν = 300 units) at room temperature. Processing of published data on the rate constant of the reaction of styrylperoxy radicals with 2,5 dichlorohydroquinone in the frame work of the intersecting parabolas method gave the O-H bond dissociation energy in 2,5 dichlorohydroquinone: D OH = 362.4±0.9 kJ mol -1 . Taking into account these data, the O-H bond dissociation energy in the 2,5 dichlorosemiquinone radical was found: D OH = 253.6±1.9 kJ mol -1 .

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