A series of batch experiments were conducted to investigate the effects of the HCl-pretreated concentrations, enzyme hydrolysis time and temperature, the cellulose dosage, the ultrasonic time and the fermentation substrate concentration on hydrogen (H(2)) production from the anaerobic fermentation of apple pomace (AP). The natural mixed microorganisms from river sludge was used as the seed after being boiled for 15 min. A maximum cumulative H(2) yield (CHYm) of 134.04 ml/g total solid (TS) and an average H(2) production rate (AHPR) of 12.00 ml/g TS/h were obtained from the fermentation of the enzyme-hydrolyzed AAP (a AP soaked with 6% ammonia liquor for 24 h) at the substrate concentration of 15 g/L. The optimal enzyme hydrolysis conditions were proposed as follows: AAP with a cellulase dosage of 12.5 mg/g TS at the hydrolysis substrate concentration of 20 g/L after the ultrasonic irradiation for 20 mm was hydrolyzed by the enzyme catalysis at 45 degrees C and initial pH 5.0 for 48 h. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.