Acidification of simulated dairy wastewater was conducted in an upflow reactor at 55 degrees C. Results showed that the degree of acidification decreased with the increase in chemical oxygen demand (COD) loading rate, from 60.8% at 4 g l(-1)day(-1) to 27.1% at 24 g l(-1) day(-1). Carbohydrate was readily degraded at all loading rates, but degradation of protein and lipid decreased with the increase in loading rate. Most carbohydrate degradation occurred at the reactor bottom, whereas protein was degraded mainly after the carbohydrate became depleted. The predominant acidification products were acetate, propionate, butyrate and ethanol, whereas formate, i-butyrate, valerate, i-valerate, caproate, lactate, methanol, propanol and butanol were present in lesser quantities. The increase in loading rate resulted in the increase of propionate and the decrease of acetate, but had little effect on ethanol and butyrate productions. Only 2.5-8.8% of influent COD was converted to hydrogen and methane. The biomass yield was mg(-1) GOD.