This study explored the effect of pH on the acid-phase anaerobic digestion of primary sludge. Controlled and uncontrolled pH experiments were conducted using two bench-scale, continuous-flow reactors having different configurations : a completely mixed reactor (CMR) with clarifier and solids recycling, and an upflow anaerobic sludge blanket (UASB) unit. Results indicate that the specific rates of VFA production and COD solubilization, in either system, were not affected by the variation in pH between 4.3 and 5.2, but at higher pH values (5.9-6.2) a significant decline (by 25-30%) in both parameters was observed. Analysis of the degradation behavior of the three important organic classes (carbohydrates. proteins, and lipids) revealed that each class followed an individual trend with respect to pH changes. Acetic acid was the major end-product of acidogenic digestion in both reactors, regardless of pH. The percent VFA distribution did not appear to be influenced by pH variation, except for propionic and butyric acids. Besides VFAs, small amounts of formic acid, ethanol, and lactic acid were also formed at all pH values tested.