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Differential scanning calorimetry (DSC) and thermogravimetry (TG) studies of humic acids (HA) sequentially extracted at pH 7, 8, 9 and 10 by phosphate and pyrophosphate buffers have been carried out. Supplement information were provided by elementary analysis and FTIR spectroscopy to characterize and evaluate in details the thermooxidative behavior of HA fractions. The exothermic DSC profiles of the HA samples extracted by phosphate buffers or by pyrophosphates were shifted to lower temperatures in comparison with the original sample and showed narrow distribution of humic molecules. Experimental data showed dependency of HA fraction composition on the pH used during extraction. DSC experimental data showed 1 endothermic and from 2-5 exothermic peaks associated with mass loss recorded by TG measurement. Higher degree of aliphaticity observed on samples extracted at lower pH did not remarkably affect the DSC peak temperature of the first decomposition step associated with aliphatic moieties and functional groups decomposition and recombination reactions. On the other hand, such dependency was observed in higher temperature range where destruction of aromatic structures took part. The behavior of HA samples (even after separation of specific fractions) showed high complexity of system, which resulted in complicated physical and chemical processes occurred during thermooxidation. Observed stability and released heat were affected by many factors, among them evaporation of volatile part of humic matter and a slight contribution of weak interactions among humic molecules were considered. [ABSTRACT FROM AUTHOR]