Batch and continuous experiments were conducted to study the influence of dairy wastewater strength (2-30g-COD/L) on acidification at pH 5.5 and 37 degrees C. Results of batch experiments showed that carbohydrate was preferentially acidified as compared to protein and lipid. Production of VFAs (mainly acetate, propionate and butyrate) and hydrogen corresponded to carbohydrate acidification, whereas production of alcohols (mainly ethanol, propanol and butanol), plus i-butyrate and higher molecular-weight VFAs, corresponded to protein acidification. In treating high-strength wastewaters (8-30 g-COD/ L), acetate, butyrate and P(H2) decreased after reaching their peak levels before leveling off. Results of continuous experiments with 12h of hydraulic retention showed that acidification decreased with the increase of wastewater COD, from 57.1% at 2 g-COD/L to 28.8% at 30 g-COD/L; among the constituents in dairy wastewater, 92-99% of carbohydrates, 59-85% of protein and 12-42% of lipid were acidified. High-strength wastewater favored production of hydrogen and alcohols, especially propanol and butanol. The biomass yield was 0.258g-VSS/g-COD.