An anaerobic contact reactor (ACR) system comprising a continuous flow stirred tank reactor (CSTR) with settler to decouple the hydraulic retention time (HRT) from solids retention time (SRT) was developed for fermentative hydrogen production from diluted molasses by mixed microbial cultures. The ACR was operated at various volumetric loading rates (VLRs) of 20–44 kgCOD·m ·d with constant HRT of 6 h under mesophilic conditions of 35°C. The SRT was maintained at about 46–50 h in the system. At the initial VLR of 20 kgCOD·m ·d , the hydrogen production rate dropped from 22.6 to 1.58 L·d as the hydrogen was consumed by the hydrogentrophic methanogen. After increasing the VLR to 28 kgCOD·m ·d and discharging the sludge for 6 consecutive times, the hydrogentrophic methanogens were eliminated, and the hydrogen content reached 36.4%. As the VLR was increased to 44 kgCOD·m ·d , the hydrogen production rate and hydrogen yield increased to 42.1 L·d and 1.40 mol H ·molglucose-consumed , respectively. The results showed that a stable ethanol-type fermentation that favored hydrogen production in the reactor was thus established with the sludge loading rate (SLR) of 2.0–2.5 kgCOD·kgMLVSS ·d . It was found that the ethanol increased more than other liquid fermentation products, and the ethanol/acetic acid (mol/mol) ratio increased from 1.27 to 2.45 when the VLR increased from 28 to 44 kgCOD·m ·d , whereas the hydrogen composition decreased from 40.4% to 36.4%. The results suggested that the anaerobic contact reactor was a promising bioprocess for fermentative hydrogen production.