Biohydrogen production from palm oil mill effluent (POME) through dark fermentation process was evaluated in a two different lab scale reactor configuration namely up-flow anaerobic sludge blanket fixed-film reactor (UASFF) and a continuous stirred tank reactor (CSTR). The effect of different organic loading rate (OLR) and hydraulic retention time (HRT) on hydrogen yield and volumetric hydrogen production rate (VHPR) was investigated. The data on the UASFF reactor is based on the start-up study published research data by the authors undertaken from 2021. In that study, UASFF reactor operated at large OLRs of 5-80 g COD/L.d and HRTs of 6-24h, and CSTR reactor at OLRs of 15-100 g COD/L.d and HRTs of 6-24h. The UASFF achieved a VHPR of 1.9-4.8 ± 0.1 L H2/L-d and COD removal of 35% while the CSTR had a VHPR of  1.8-4.5 ± 0.1 L H2/L-d and COD removal of 30%. The results indicated the change of bacterial community diversity over the operation in the UASFF reactor, in which Clostridium sensu stricto 1 and Lactobacillus species contributed to hydrogen fermentation. While the CSTR resulted in higher population of Clostridium sensu stricto 12 but also includes Lactobacillus sp. that mainly contribute to hydrogen production. Both Clostridium sensu stricto 1 (C. acetobutyricum) and Clostridium sensu stricto 12 (C. tyrobutyricum) are known to be producers of hydrogen as a byproduct of both respective metabolic pathways that converts carbon sources into acetic and butyric acid. Previous research has also shown that depending on the operating conditions and parameters of the fermentation process, Clostridium sensu stricto 12 can be converted into Clostridium sensu stricto 1. The similar performance of the UASFF and the CSTR can be explained by the similarity in the microbial community composition, which shows high prevalence of hydrogen-producing bacteria, Clostridium sensu stricto 1 in the UASFF and Clostridium sensu stricto 12 in the CSTR. The difference in COD removal could be due to the method operation between the two reactors, as mixing capabilities of UASFF (through up-flow velocity and recirculation rate from a settling tank to the main UASB reactor) is well-established and known to be efficient as compared to CSTR. However, CSTR can be easily modified to optimize mixing capabilities with changes in tank size, stirrer type, and also stirring speed, not to mention maintenance for CSTR is relatively simple and does not involve complex techniques.