The purpose of this study was to evaluate the effect of pH and inorganic nutrient supplementations for anaerobic hydrolysis and acidogenesis of particulate organic materials at both mesophilic (35 degreesC) and thermophilic (55 degreesC) temperatures. Hydrolysis and acidogenesis of a synthetic sludge was observed in batch operation for the evaluation of the pH effect. pH was uncontrolled in one reactor and controlled at 4.5, 5.5, and 6.5 in the other three reactors at both temperatures. The greatest degree of hydrolysis and acidogenesis occurred when the pH was controlled at 6.5. The pH of the uncontrolled reactor dropped to 3.4 at both temperatures severely retarding hydrolysis and acidogenesis. Concentrations of acetic and n-butyric acids predominated with lower concentrations of propionic acid at both temperatures in all reactors. Lactic acid was produced as the earliest intermediate but as the reaction proceeded, short chain VFAs were produced as final end products with a decrease in lactic acid. The higher the pH, the earlier this trend was observed. For the controlled reactors at pH 6.5, the soluble COD production and the VSS reduction peaked in 4 days at 55 degreesC whereas it took about 11 days at 35 degreesC to obtain the same result. During the linear SCOD production period at a pH of 6.5 the hydrolysis rate of the thermophilic reactor was greater than that for mesophilic. Thermophilic conditions appeared to be more sensitive to pH than mesophilic ones for both hydrolysis and acidogenesis. Additional experiments were conducted to establish the effect of inorganic nutrient (Ca, Fe, Co, and Ni) supplementation on hydrolysis and acidogenesis at both temperatures. it has, prior to this, been assumed that only methanogenesis benefited from trace metal supplementation. However, the results demonstrated the importance of inorganic nutrient supplementation to optimize hydrolysis and acidogenesis at both temperatures.