Enhancement of biological H-2 production efficiency with pre-ozonation process of palm oil mill effluent (POME) prior to thermophilic dark fermentation (55 degrees C) was investigated. H-2 fermentation experiments were conducted using varying concentrations of raw and ozonated POME. Results revealed that H-2 can be produced from both raw and ozonated POME under thermophilic fermentation. Maximum H-2 production yield of 77 mL.g(-1)COD(removed) was obtained from ozonated POME, which was higher than that of 51 mL.g(-1) CODremoved obtained from raw POME at the highest concentration of 35,000 mg COD.L-1. Meanwhile, the specific H-2 production rate (R'(max)) of 1.9 and 1.5 mL.h(-1).g(-1) TVS were observed in raw and ozonated POME at the concentration of 25,000 mg COD.L-1, respectively. The main metabolic products during POME fermentation were acetic and butyric acids and trace amount of valeric acid. Propionic acid and ethanol have contributed, which could be reduced H-2 production in all batch experiments for both POME. The highest efficiency of total and soluble COD removal of 24 and 25% was obtained from the raw POME, and those of 19 and 25% was obtained from the ozonated POME. The present study demonstrates that the POME loading was greatly influenced on the H-2 production yields and rates. The comparative results showed that the ozonated POME gave higher H-2 yields than the raw POME. Thus, demonstrating that the ozonation process significantly improved the POME biodegradability, which is able to enhance H-2 production yields. However, the ozone pretreatment was not improved in the specific H-2 production rates. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.