Biological H-2 production from anaerobic digestion (AD) is not a sustainable pathway, based on both the efficiency of energy recovery and operation possibility in practice. Because of this, the use of AD for producing CH4 ought once again to be considered and some exopathic measures should be studied to enhance CH4 production. Among these, the intervention of exogenous CO2 and/or H-2 has been demonstrated as an external approach to stimulating CH4 production. When exogenous H-2/CO2 was intervened in mesophilic AD, the H-2 consumed pathways were found to change due to increased H-2 partial pressure (P-H2) in this study. At P-H2 = 0.96 bar, hydrogenotrophic methanogens (HMs) and homoacetogens (HAs) contributed 60% and 40% respectively to the H-2 consumption, which resulted in an increased CH4 production via the combination of homoacetogenesis and aceticlastic methanogenesis. The enumeration experiments determined that the amount of HMs was two orders higher in magnitude than that of HAs, and that K-s (half-velocity constant) and V-max (maximum specific substrate utilization rate) of HAs were respectively 10 times higher and 4 times lower than those of HMs. The determined kinetic parameters reveal that a low P-H2 as in normal AD makes HAs contribute much less (2-5%) to the H-2 consumption. Based on the fact that there was an upper limit to the capacity of HMs for the H-2 consumption due to their limited amounts in the reactor, any attempts to inhibit the pathway of HAs for H-2 consumption would have no use in technology. (c) 2015 Elsevier B.V. All rights reserved.