Batch tests were conducted to select a good natural mixed microflora seed source and determine its optimal initial cultivation pH to elevate hydrogen production from xylose (20 g-COD/L). Hydrogen production efficiency was evaluated using the hydrogen yield (HY) and maximum hydrogen production rate (HPRm). Sewage sludge was shown to have higher HY and HPRm than other biological sludges collected from distillery, food and paper mill wastewater treatment plants. The experiments of pH effect were conducted at pH 5-8 and 35 degrees C. The experimental results indicate that the initial cultivation pH markedly affected HY, HPRm, liquid fermentation product concentration and distribution, butyrate/acetate concentration (HBu/HAc) ratio and metabolic pathway. For sewage sludge, its optimal initial cultivation pH for hydrogen production was 6.5 with peak values of HY 1.3 mol-H-2/mol-xylose, HPRm, 0.25 mol-H-2/L-day, butyrate concentration 5780 mg-COD/L and HBu/HAc ratio 4. However, a pH change of 0.5 units from pH 6.5 decreased hydrogen production efficiency by 23%. Strategies based on the experimental results for optimal hydrogen production from xylose are proposed. (c) 2006 Elsevier Ltd. All rights reserved.