Temperature affects H2 production, metabolite product distribution, substrate degra­dation and bacterial growth. Most studies on H2 production have been conducted under ambient (15-27°C), mesophilic (30-45°C), and moderate thermophilic (50-60°C) temperatures, with a few studies of mixed cultures under extreme thermophilic conditions, over 60°C [88, 89]. The optimal temperature for H2 production via dark fermentation varies widely based on the type of biocatalyst and the carbon substrate used. For pure cultures, the optimal temperatures are reported to be in the range of 37-45°C, whereas for mixed microflora diverse optimum tem­peratures were reported [62]. Both mesophilic and thermophilic temperatures were observed to be optimal for fermentative H2 production processes. Thermophilic conditions were reportedly advantageous due to its thermodynamics [15, 62] which gives higher reaction rates with better process performance and decreased problems with contaminating H2-consuming microorganisms. Although higher temperatures allow more favorable reaction kinetics, rapid changes in system pH may inhibit H2 producing bacteria [90]. The changes in soluble metabolite composition were also observed with changes in operating temperature, resulting in metabolic pathway shifts correlated to bacterial functions dominant at that particular temperature [91]. Temperature control might not be a feasible option for process control.