Abstract Hydrogen is a clean fuel which can be burnt without any carbon emissions. To minimize greenhouse gas emissions, humanity would need a significant amount of hydrogen as a fuel. The goal of this work is to understand the mechanisms of geological production of hydrogen in mafic and ultramafic rocks. Hydrothermal reactions of peridotite rocks were conducted in stirred-tank and fixed-bed reactors. The effect of pH was experimentally studied at 250 °C and 1200 psi. A microGC with a TCD detector was used to analyze the produced H2. The rocks were analyzed by SEM-EDS, XRF, and XRD before and after the reaction. The cations (Ca2+, Fe, Mg2+, and Na+) in the water were analyzed by ICP-OES. The hydrothermal reaction produced a significant amount of H2, which varied with pH, with a minimum at the neutral pH and a maximum at the acidic conditions (0.80 ± 0.17 moles H2/kg of rock in 13 days). A fixed-bed reactor of peridotite at 250 °C and 1200 psi yielded much less H2 production after 2 pore volume of injection in alkaline solutions. The reaction has a minimum rate at the neutral pH and is mass transfer limited.