Abstract Geological sources of H 2 and abiotic CH 4 have had a critical role in the evolution of our planet and the development of life and sustainability of the deep subsurface biosphere. Yet the origins of these sources are largely unconstrained. Hydration of mantle rocks, or serpentinization, is widely recognized to produce H 2 and favour the abiotic genesis of CH 4 in shallow settings. However, deeper sources of H 2 and abiotic CH 4 are missing from current models, which mainly invoke more oxidized fluids at convergent margins. Here we combine data from exhumed subduction zone high-pressure rocks and thermodynamic modelling to show that deep serpentinization (40–80 km) generates significant amounts of H 2 and abiotic CH 4 , as well as H 2 S and NH 3 . Our results suggest that subduction, worldwide, hosts large sources of deep H 2 and abiotic CH 4 , potentially providing energy to the overlying subsurface biosphere in the forearc regions of convergent margins.