Application of ultrasonic fields with a power of 300 W and the frequency 25 kHz, causes changes in the sewage sludge and in content of volatile fatty acids produced during the sludge anaerobic digestion. These changes depend on the applicable sonification time. The most favourable time was assumed to be 15 minutes. The differences have been observed at concentrations of volatile fatty acids generated, although 1/5 of the sludge volume was sonificated. This resulted in an average 15% increase in VFA content compared to non-sonificated sludge. At the same time, a higher degree of sludge organic matter decomposition was obtained, which had a positive effect on the sludge stabilization. The sludge disintegration via sludge sonification caused also a higher reduction in sludge weight (total solids).