Experiments were conducted to study the acidogenesis of a dairy wastewater in batch reactors at pH 5.5 and 55 degrees C. There was a biased fermentation sequence for carbohydrate and protein, and the protein fermentation was delayed by carbohydrate. The production of hydrogen was exclusively from the fermentation of carbohydrate. Acetate and butyrate concentrations both increased rapidly at the beginning and peaked at some points, then declined in the reactors fed with 8 g-COD (chemical oxygen demand)/l, or higher concentrations. Butanol and propanol fractions increased with the substrate concentration. The metabolism shifted from the volatile fatty acid-producing pathways to the alcohol-producing pathways when the substrate concentration increased beyond 8 g-COD/l. The acidogenic biomass yield was in the range 0.19-0.25 mg-VSS/mg-COD.