Electrochemical studies with nanocrystalline nickel (grain size ≈ 100 nm) have shown that hydrogen permeation is remarkably larger in comparison with microcrystalline nickel (grain size ≈ 2 μm). This is due to an increase of both hydrogen solubility and hydrogen diffusivity. The latter quantity increases by two orders of magnitude when the hydrogen activity is enlarged. However, measurements of the time lag during transient permeation show that at very low hydrogen activities the diffusion coefficient can be smaller when compared to a microcrystalline sample. For small cathodic current densities (< 10 μA/cm2) all of the produced hydrogen is absorbed in atomic form by the sample whereas for large current densities the overwhelming part of the electric charge is consumed for the formation of gaseous hydrogen according to the Volmer-Tafel mechanism. The results are explained in the framework of a model used for hydrogen diffusion in defective and disordered materials. © 1993.411132153222Windle, Smith, The Effect of Hydrogen on the Plastic Deformation of Nickel Single Crystals (1968) Metal Science, 12, p. 187Windle, Smith, The Effect of Hydrogen on the Deformation and Fracture of Polycrystalline Nickel (1970) Metal Science, 14, p. 136Latanision, Kurkela, (1983) Corrosion, 39, p. 174Yao, Cahoon, (1991) Acta metall. mater., 39, p. 119Harris, Latanision, (1991) Metall. Trans. A, 22 A, p. 351Mütschele, Kirchheim, Segregation and diffusion of hydrogen in grain boundaries of palladium (1987) Scripta Metallurgica, 21, p. 135Moody, Foiles, (1991) Structure/Property Relationships for Metal/Metal Interfaces, 229, pp. 179-185. , 2nd edn., A.D. Roming, D.E. Fowler, D.P. Bristowe, MRS, Pittsburgh, PaDevanathan, Stachuski, The Adsorption and Diffusion of Electrolytic Hydrogen in Palladium (1962) Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 270 A, p. 90. , 2nd ednKirchheim, Hydrogen solubility and diffusivity in defective and amorphous metals (1988) Progress in Materials Science, 32, p. 262Iyer, Pickering, (1990) A. Rev. Mater. Sci., 20, p. 299Boes, Züchner, (1976) J. less-common Metals, 49, p. 223D. Arantes and R. Kirchheim, to be publishedKaesche, (1979) Die Korrosion der Metalle, p. 79. , 2nd edn., Springer, BerlinMütschele, Kirchheim, Hydrogen as a probe for the average thickness of a grain boundary (1987) Scripta Metallurgica, 21, p. 1101Kirchheim Kownacka, Filipek, Hydrogen segregation at grain boundaries in nanocrystalline nickel (1993) Scripta Metallurgica et Materialia, 28, p. 1229Kirchheim, Stolz, (1985) J. non-cryst. Solids, 70, p. 323Robertson, (1973) Z. Metallk., 64, p. 436Kaur, Gust, (1988) Fundamentals of Grain and Interphase Boundary Diffusion, , Ziegler, Stuttgart