To quantify the H2 production associated with serpentinization at slow spreading mid-oceanic ridges, hydrothermal alteration experiments have been performed. Starting materials are a fine-grained harzburgite from Pindos ophiolite and artificial seawater (3.2 wt.% NaCl). Two grain sizes (P1 and P2), both <60μm, are used. The water-rock ratio is ~1.8 (40g of rock and 72g of water). Experiments are carried out at 250, 300 and 350°C, and for a constant pressure of 500 bar. For each temperature, two experiments are conducted in parallel. In the first, the hydrothermal fluid is periodically sampled and its H2 concentration ([H2]) determined by gas chromatography. In the second, the H2 fu/gacity (ƒH2) inside the reaction cell is continuously monitored with a H2semi-permeable membrane made of Au20Pd80. [H2] is maximum at 300°C with a plateau at ~115 mmol/kgwater against ~50 mmol/kgwater at 250°C (for the same duration, the experiment at 350°C (still in progress) shows [H2] ~10 times lower). The kinetics of H2 production is controlled by the particle size. At 300°C, the [H2] plateau is reached in ~35 days for P2 against ~55 days for P1. At 250 and 300°C, an ƒH2 increase is observed after the 2nd day and the rate of fH2 increase reaches a maximum during the 4th day. At 300°C, the ƒH2 stabilizes after ~40 days at 23 bar (fO2 ≈ NNO - 4.1 to 4.3). At 250°C, it takes ~70days to reach a plateau ƒH2 of 38 bar (fO2≈ NNO - 4.6 to 5.1). The solid products recovered and analyzed at the end of experiment are characterized by a serpentinization rate > 60% and they contain variable amounts of magnetite and brucite, in addition to serpentine minerals. To link ƒH2 and [H2], experiments coupling the two types of measurement have been performed in the H2O-H2 system. For [H2] <50 mmol/kgwater, at 250 and 300°C, the first results show ƒH2/[H2] ≈ 660 and 380 bar.kgwater/molH2respectively. These results imply that H2-bearing fluids associated with serpentinization strongly positively deviate from ideal behaviour, consistent with the proximity of the two-phase field of the H2O-H2 system. This specific behaviour of H2O-H2 fluid mixtures needs to be accounted for in thermodynamic models of serpentinization.