Ozone pretreatment of palm oil mill effluent (POME) was employed to improve sustrate biodegradability prior to biological H-2 production. The H-2 production was conducted at varing pHs from 4.0 to 6.0 to examine the impact of pH on the H-2 mesophilic production (37 degrees C). The optimal pH for H-2 production was 6.0 for both raw and ozonated POME. The POME concentrations were greatly influenced the yields and rates of H-2 production. At the optimal pH, the maximum H-2 production yield of 182 7.2 mL.g(-1) COD (7.96 mmoL.g(-1) COD) was achieved at the ozonated POME concentration of 30,000 mg COD.L-1. The maximum H-2 production rate (Rma.) of 43.1 2.5 mL.h(-1) was obtained at the ozonated POME concentration of 25,000 mg COD.L-1. The highest total COD removal was 44% at of 15,000 mg COD.L-1 ozonated POME. Acetic and butyric acids were dominant products during H-2 fermentation and tended to increase with the increased POME concentrations. Ozonation as a pretreatment process showed significant enhancement of the POME biodegradability, and subsequently improved the H-2 production H-2. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.