There have been no reports of H-2 and CO2 mixed-culture fermentation (MCF) at 25 degrees C in a hollow-fiber membrane biofilm reactor (HfMBR). In this study, H-2 and CO2 MCF were conducted in an HfMBR at 25 degrees C producing metabolites including acetate, ethanol, butyrate, and caproate. Compared to pure culture fermentation (i.e., Clostridium carboxidivorans P7), the MCF in HfMBR at 25 degrees C produced a higher concentration of caproate in this study (3.4 g/L in batch 1 and 5.7 g/L in batch 2). The dominant genera were Clostridium_sensu_stricto_12 and Prevotella_7. The caproate was more likely formed from the pathway of acetate and ethanol rather than via butyrate and ethanol. Since caproate is more valuable than acetate and low temperature fermentation consumes less energy, this process of H-2 and CO2 MCF at 25 degrees C is appropriate for industrial application.