The effect of pH on anaerobic solubilization of synthetic sludge (dog food) and domestic primary sludge was investigated and compared. Anaerobic solubilization was carried out in Continuously Stirred Anaerobic Reactors at mesophilic temperature (35 degrees C) and pH was fixed at 6.5 (pH-controlled). The aim of pH control in the reactors consisting of synthetic and primary sludge, was the evaluation of retardation in hydrolysis/acidogenesis at low pH values. Since synthetic and primary sludge have different biodegradation characteristics, the results were compared. In both sludges, acetic acid was the main Volatile Fatty Acids (VFAs) produced. Volatile Suspended Solid (VSS) reduction was found as 67% in about 20 days in the anaerobic digestion of synthetic sludge. whereas for the same interval VSS reduction could only be achieved by 32% in primary sludge at 35 degrees C in the pH-controlled reactors. When both types of sludges were used as substrates, the reactors removed VSS with a corresponding production of VFAs and Soluble Chemical Oxygen Demand (SCOD). However, in the pH-controlled reactors production of VFAs and SCOD was ceased after 5 days in primary sludge whereas VFAs and SCOD production continued after 5 days in synthetic sludge which indicated that hydrolysis and fermentation in the anaerobic solubilization reactors were not complete and continued longer. On the other hand, in the pH-controlled reactor of primary sludge, methanogenic phase could operate after 5 days of operation as hydrolysis/acidogenesis stopped.