After the closure of the underground nuclear waste repository, aqueous corrosion of the steel canister and, to a lesser extent, radiolysis of water would produce significant amounts of H2. This H2 can interact with materials from the repository and with the surrounding clay host formation. The COx formation contains pyrite (FeS2), which has been demonstrated to react with Hydrogen gas (H2) (Truche et al. 2010) at temperature ranging from 90°C to 180°C. This work aims at understanding these interactions at 25°C. With regards to E-pH equilibrium diagrams at 25°C for the two systems S-H2O and Fe-S-H2O with a total dissolved S concentration of 0.1 mole S per liter (about pH2S=1bar), FeS2 must be an oxidant for H2 at pH higher than 9, and FeS2 should transform into Pyrrhotite (FeS1-x), according to: FeS2 + (1-x) H2 = FeS1+x + (1-x) H2S (with 0