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Serpentinization reaction is known as one of the main sources of natural H2 at the Earth’s surface. Estimates of H2 production during this reaction require an in-depth understanding of the mineralogical processes leading to iron oxidation. The study of serpentinized peridotites collected at 13 localities at mid-ocean ridges, in ophiolites and ultramafic bodies reveals the development of an alteration sequence during reaction. At the olivine contact, a first reaction zone is composed of a fine-grained mixture of serpentine, Fe-brucite and awaruite (Reaction Zone 1). Thermodynamic modelling with the latest data for the Fe(OH)2 endmember indicates that awaruite formation limits H2 production with H2 concentrations comprised between 10-3 and 10-2 mol/kg. These values are consistent with the maximum values measured in fluids expelled at ultramafic-hosted hydrothermal sites. At the mesh rim, a second alteration zone composed of Ni-bearing magnetite, serpentine and Mg-brucite is found (Reaction Zone 2). Serpentine and Mg-brucite display a porous symplectite microtexture, indicating formation after Reaction Zone 1 by a dissolution-precipitation process. Magnetite formation in Reaction Zone 2 could not be reproduced with thermodynamic modelling by modifying, as previously thought, temperature or water to rock ratio. However, removing H2 from the system was found to reproduce both the mineralogy and the composition of Reaction Zone 2. This indicates that H2 diffusion is the main driver for magnetite formation during serpentinization. The H2, aq concentrations at the equilibrium with Reaction Zone 2 fall in the 10-7 - 10-3 mol/kg range. Based on the mineralogical observations and thermodynamic modelling performed here, two regimes for H2 production during olivine serpentinization can be proposed. If H2 diffusion is limited, the serpentinizing fluid contains between 10-3 and 10-2 mol/kg of H2 but the overall H2 production is one order of magnitude smaller than previous estimates. If H2 diffusion proceeds rapidly, the overall H2 production is comparable to previous estimates but the expected H2 concentration in the serpentinization fluid at the equilibrium with the reaction products is extremely low (10-7 to 10-3 mol/kg).