In solid-state anaerobic digestion, so-called ss-AD, biogas production is inhibited at high total solids contents. Such inhibition is likely caused by a slow diffusion of dissolved reaction intermediates that locally accumulate. In this study, we investigated the effect of H-2 and CO2 partial pressure on ss-AD. Partial pressure of H-2 and/or CO2 was artificially fixed, from 0 to 1 557 mbars for H-2 and from 0 to 427 mbars for CO2. High partial pressure of H-2 showed a significant effect on methanogenesis, while CO2 had no impact. At high P-H2, the overall substrate degradation decreased with no accumulation of metabolites from acidogenic bacteria, indicating that the hydrolytic activity was specifically impacted. Interestingly, such inhibition did not occur when CO2 was added with H-2. This result suggests that CO2 gas transfer is probably a key factor in ss-AD from biomass. (C) 2015 Elsevier Ltd. All rights reserved.