The article presents the results of research on the impact of inorganic coagulants and polyelectrolytes and their common action on sonicated digested sludge. Sonication of sludge samples was carried out under static conditions for 60, 120 and 180 seconds. The ultrasonic wave of f = 20 kHz and two different amplitudes of A = 15.25 μm was used in tests. The coagulant PIX123 and the polyelectrolyte Zetag 8160 were used for conditioning. On the basis of CST test the doses of chemical reagents were chosen for conditioning. The results showed the effect of application of the PIX123 and Zetag 8160 and their combination on non-sonicated and sonicated sludge. The lowest CST was achieved for non-sonicated sludge while polyelectrolyte was applied. For sonicated sludge better results were achieved when PIX123 was used. The combination of coagulant and polyelectrolyte allowed to achieve COD reduction, but the best results were achieved for sludge prepared by PIX123. The application of Zetag8160 in this case was not satisfactory.