The addition of external redox mediators (e.g., humic substances) as a strategy to enhance the production of H₂ and carboxylic acids through mixed-culture dark fermentation was investigated. Pahokee Peat and Leonardite humic substances along the anthraquinone-2,6-disulfonate (AQDS) humic model compound were evaluated. The use of Pahokee peat into dark fermentation assays reached an H₂ potential of 618.2 ± 28.9 mL H₂/L, equivalent to an increase of 8% compared with the control experiment. In contrast, the addition of Leonardite (in its basal redox state) as a redox mediator increased the H₂ potential by up to 14.5% and achieved 655.8 mL H₂/L-d, which was higher than the control's H₂ production of 572.6 mL H₂/L-d. Moreover, the use of chemically reduced Leonardite led to the highest H₂ potential (1158.6 mL H₂/L-d) which was about 1.8-fold the H₂ potential found in the control. Overall, experimental and production-modeling results suggest that external redox mediators not only served as an additional source of electrons for dark-fermentative pathways but also as electron acceptors. Electrons from redox mediators were likely transported through membrane proteins and could be used to reduce NAD⁺ and Fd_ox. Results indicated that electrons were preferably channeled through the butyrate pathway when reduced substances were applied.