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The effects of operational and environmental parameters on particulate organic carbon solubilization during the acid-phase digestion of primary municipal sludge were studied using a bench-scale, upflow anaerobic sludge blanket (UASB), continuous-flow reactor. Short-chain volatile fatty acids (VFAs), the main products of the process, are preferred substrates for the bacteria involved in the biological removal of phosphorus. Both particulate organic matter solubilization [measured by filtered total organic carbon (TOC) increase or volatile suspended solids (VSS) reduction] and product formation (mainly VFAs) were affected by the changes in hydraulic retention time (HRT) from 6 to 15 h. Variation in solids retention time (SRT) from 5 to 20 days (at a constant HRT of 12 h) had a small effect on specific TOC solubilization rate and percent VSS reduction. In contrast, VFA production decreased sharply at the shortest SRT investigated. At an HRT of 12 h and a SRT of 20 days, a decrease in pH from 5.1 to 4.5 did not significantly influence system behavior. However, an increase in pH from 5.1 to 6.1 produced a higher specific TOC solubilization rate and a lower net VFA production. The settling properties of the UASB sludge blanket were excellent. Gas generation from the system was small. The amount of VFAs generated, in most runs, was large enough to support subsequent biological phosphorus removal processes.