Hydrogen (H-2) produced by dark fermentation is an alternative to fulfill the requirements of the transportation sector and to be a complementary source in the forthcoming electricity grid. However, the dark fermentative H-2 production is limited by the accumulation of H-2 in the fermentation broth. In continuous stirred-tank reactors (CSTR), such phenomenon is associated with poor mass transfer conditions. Nevertheless, this parameter has been scarcely considered to enhance H-2 production. In this research, the effect of the H-2 mass transfer conditions on the productivity and efficiency of continuous H-2 production was evaluated using a series of CSTR operated at H-2 mass transfer coefficients (kLa) ranging from 1.04 to 4.23 1/h. The results showed that volumetric H-2 production rate (VHPR) and H-2 yield increased 74 and 78%, respectively, due to enhanced mass transfer conditions. This behavior was driven by 53% decrease of the dissolved H-2 concentration. More specifically, the maximum VHPR of 7.66 L/L-d with a H-2 yield of 1.1 mol H-2/mol hexose was obtained at a k(L)a= 4.23 1/h. Furthermore, 16S-DGGE analysis and sequencing revealed that Clostridium and Lactobacillus were the dominant bacterial genera in continuous operation. In particular, Clostridium increased its occurrence at kLa of 2.72-4.23 1/h as a response to lower dissolved H-2 concentrations. The novelty of this work relies on the demonstration that mass transfer conditions control not only the H-2 accumulation and reactor performance (VHPR and H-2 yield), but they also influence the metabolic pathways and the composition of the microbial community.