Microbial activity determining the speed of the changes occurring during the process of anaerobic digestion depends on the environmental conditions of the process. The factor intensifying the process of methane fermentation is an increase of temperature. The conducting of the methane fermentation in thermophilic conditions affect the rate of the reactions taking place during the phase of hydrolysis, thus contributing to increase the concentration of volatile fatty acids generated from the sludge, resulting in increasing the production of biogas. It should be noted, however, that thermophilic bacteria have a high sensitivity to temperature changes already the order of ± 0.5 ° C/d at their optimum growth in the range of 52-55 °C. Therefore, for the process would run optimally smoothly unnecessary to ensure stable environmental conditions leading to maximum reduction of organic substances. The basic test substrate, characterized by a high content of organic and nitrogen compounds, is excess sludge. To determine the effect of generating the volatile fatty acids during the thermophilic fermentation the sludge was submitted 8-daily anaerobic stabilization. The following designations of physico-chemical characteristics were made: dry matter, pH, alkalinity, volatile fatty acids, the chemical oxygen demand. In the following days of the process operation with regard to fermentation carried out under mesophilic conditions the increase of volatile fatty acids generation was observed, which also correlated with the increase of the chemical oxygen demand. In the 8th day of thermophilic conditions the concentration of volatile fatty acids was 1749 mgCH₃COOH/dm³, while the chemical oxygen demand 3,279 mgO₂/dm³ and the alkalinity row of 3800 mgCaCO₃/dm³.