In solid-state anaerobic digestion, the production of methane is limited compared to liquid-state anaerobic digestion. One of the possible reasons of this limitation could be the reduction of the diffusion of the molecules due to the high total solid content, creating local inhibitory environment. This study investigates the effect of the thickness of the reaction environment and by extension the effect of the diffusion on solid-state anaerobic digestion to evaluate this hypothesis. Two kind of anaerobic reactors with different thicknesses of a wheat-straw based medium (1.0 and 4.2 cm) were investigated for 33 days. The results showed an inhibition of methanogenesis for 4.2 cm height, which generated a gradient of propionate all along the medium thickness. This gradient was likely due to a local accumulation of H-2, and a high partial pressure of hydrogen can result in an inhibition of the hydrolysis step. This result could explain why a high total solid content in solid-state anaerobic digestion results in an inhibition of the global anaerobic digestion, due to the local accumulation of inhibitory products. Therefore, mixing of solid-state anaerobic digester is mandatory to improve the methane production.