Several studies have reported depleted values for the hydrogen isotope signatures of mantle xenoliths (Bell & Ihinger, 2000; Yu et al., 2005; O’Leary, 2007). The deep origin of these depleted values has been questioned. We performed hydrogen-deuterium diffusion experiments in cubic pieces of natural xenolith aggregates of 1 cm3, between 600 and 900°C and at room pressure in deuterium enriched gas. The objective was to explore the role of grain boundary diffusion in the control of isotopic exchanges during xenolith transport up to the surface. We compared intracrystalline diffusion profiles measured in orthopyroxenes (opx) single crystals and opx located at the edge of the cubes with diffusion profiles within opx inside the cubes. The diffusion profiles in crystals inside the cubes are only slightly shorter than the ones in crystals directly in contact with the gas. In the experiments, apparent diffusion coeffcients in opx inside the cubes are lower by a maximum of 0.6 log units. It suggests that the grain boundaries diffusion of hydrogen in xenoliths is at least 2.5 log units faster than intracrystalline diffusion in opx. The results confirm that the isotopic exchange is fast enough to constantly requilibrate the δD values between the xenolith and the surrounding magma. δD depletion in pyroxenes from mantle xenoliths may be strongly influenced by late degasing prior and during eruptions. Bell, D.R., Ihinger, P. D., (2000). The isotopic composition of hydrogen in nominally anhydrous mantle minerals. Geochimica et Cosmochimica Acta, 64(12), pp.2109-2118. Yu H., Xia, Q., Deloule, E., Yang X., (2005). Hydrogen Isotopic Compositions of Pyroxenes in Nushan Peridotite Xenoliths, SE China. Acta Geologica Sinica - English Edition, 79(3), pp.336-342. O’Leary J.A., (2007). Hydrogen isotope geochemistry of the mantle: constraints from back arc basin basalts and mantle xenoliths, PhD Thesis, California Institute of Technology, pp. 137.