Simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) of different lake aquatic humic-solute fractions were investigated under atmospheric temperature and pressure. The samples were subjected for the destruction of organic matter within the 105-600 degrees C range at a rate of 5 degrees C/min. For each different humic-solute fraction, four-to-five partially overlapping thermal degradation stages were obtained from the shapes of the TG curves. A first-order approximation was applied to estimate the energy of activation for different multi-step degradation stages. A powerful reaction took place at 370-500 degrees C for every acidic humic-solute fraction which indicated the decomposition of a very stable structural 'nucleus' with relatively strong bond energies. The results of TG-DTA demonstrate that the combination of the four diverse humic-solute pieces, isolated/fractionated by the XAD-technique in strongly acidic conditions, consisted of structural subunits the behaviour of which resembled that of the bulk of humic solutes isolated by the DEAF procedure. Apart from this, the diverse humic-solute fractions do not occur wholly independently in the bulk but are, to some extent, aggregated via different bondings.