Acid-phase fermentation of municipal primary sludge as well as industrial wastewater from a potato processing plant (which is rich in starch) was investigated under different operational and environmental parameters. The operational parameter investigated was hydraulic retention time (HRT) and the environmental one was temperature. Moreover, the effect of addition of the industrial wastewater to the primary sludge on acidogenesis was also examined. The study was carried out using two 2-kiter completely mixed reactors, each with a clarifier and a recycle facility. One of the reactors was fed with a 1:1 ratio (by volume) mixture of municipal primary sludge and industrial wastewater, while the other was fed with primary sludge alone. HRT was selected at 18 and 30 hours, while three different temperatures of 22$\sp\circ$C, 30$\sp\circ$C and 35$\sp\circ$C were used. The addition of the industrial wastewater and the operational and environmental parameters investigated had profound consequences on the acid-phase digestion process. (Abstract shortened by UMI.)