The article presents the study on the effect of thermal and acidic hydrolysis treatment on the antioxidant activity of humic acids. It clarifies the changes in macromolecule structure resulted from the variations in periphery/nucleus ratio. Using the differential-thermal analysis, kinetic parameters of humic acid thermal-oxidative destruction were determined. The studied samples of humic acids were divided into three groups: humic acids extracted from air-dried fen and terrestrial fuscum peat, humic acids subjected to the two-stage hydrolysis with 10% solutions of HCl and HF, humic acids thermally treated in decomposing gases of fen and terrestrial fuscum peat and subjected to the two-stage acidic hydrolysis. The relationship between DSC, DTG, TG curves characteristic parameters and peculiarities resulted from the treatment of humic acid macromolecule was established. The kinetic parameters of thermal-oxidative destruction within two temperature regions were defined: low-temperature region 150-380 °С and high-temperature region 380-700 °С. Z coefficient that allowed defining the number of low-molecular fractions in humic acid macromolecule was calculated. It was proved that treatment of humic acid increases its antioxidant activity.