As abiotic hydrogen redox reactivity is kinetically limited, most of the possible redox reactions induced by hydrogen remain insignificant at low temperature, even at a geologic time scale. This is the case for sulphate and carbonate reduction, but some H-2 induced redox reactions may be significant at low temperature conditions, in particular the pyrite reduction into pyrrhotite. In this experimental study, the geochemical impact of hydrogen in a clay-rich rock containing 1-2 wt.% framboidal pyrite is evaluated under mid-hydrothermal condition (90 to 250 degrees C) and with hydrogen partial pressure ranging from 3 to 30 bar. This study demonstrates that the main geochemical perturbation induced by hydrogen in a claystone host-rock formation is the destabilisation of pyrite, which leads to the production of sulphide. Two different reaction mechanisms can be distinguished as a function of temperature and hydrogen pressure. When temperature is lower than 150 degrees C and the hydrogen partial pressure is below 6 bar, pyrite solubility controls the sulphide concentration at low values. By contrast, at higher temperature or at higher hydrogen partial pressure, the rate and extent of the reaction are driven by pyrrhotite precipitation. A complete replacement of pyrite into pyrrhotite occurs within a short interval of time at T > 90 degrees C and P(H-2) > 10 bar. The pH of the media is also a critical parameter controlling the extent of the reaction as alkaline conditions may promote pyrrhotite precipitation at lower temperature and hydrogen pressure. These conclusions have a direct application in the safety assessment of nuclear waste repositories, but may also be extended to other contexts such as the underground storage of hydrogen, and, given the wide range of temperatures used in the experiments, can even be applied to hydrothermal-hosted submarine systems where hydrogen is naturally produced by olivine serpentinization. (c) 2013 Elsevier B.V. All rights reserved.