Thermal methods of sludge disintegration can be divided into high temperature (above 100°C) and low temperature (below this temperature). They consist in the supply or removal of thermal energy, contributing to changes in sludge structure and physico-chemical properties. During chemical disintegration of excess sludge with sodium hydroxide, there is an increase in the pH value, as well as changes in their structure. Ions OH- are highly toxic to microorganisms living in the excess sludge and affect the decline of biological activity of the most microorganisms. The aim of the conducted research was to evidence the impact of the thermal and alkaline disintegration of excess sludge on the susceptibility of organic substances to biodegradation. Thermal disintegration of excess sludge was carried out in a shaking water bath, in which the sludge placed in laboratory flasks with an active volume of 0.5 L were heated for a specified period within the scope of the so-called low temperatures i.e. 65÷95°C. The sludge was heated for a period of 0.5÷12h. The alkaline disintegration of the sludge was carried out with sodium hydroxide in the form of dust at ambient temperature, in sealed bottles with an active volume of 5L made of plastic, the contents of which were mixed every few hours by hand. Regent doses in the range of 0.05 ÷ 1.3 g NaOH/g VSS and disintegration time 12h were used. As a result of subjecting excess sludge to disintegration by selected methods, an increase in the concentration of organic substances in the dissolved form in the supernatant liquid was noted. Based on the increase of SCOD, TOC value and VFAs concentration, the most favorable modification conditions were determined. As a result of disintegration of the sludge and subsequent methane fermentation, supporting effects of the applied modification methods were observed, in relation to the conventional methane fermentation of excess sludge.