Micromechanisms are considered as the so called "mechanical instability" (Int. Sci. J. Alternative Energ. Ecol. 2 (2002) 36) of some solids (metallic materials and compounds) in hydrogen atmosphere or under electrolytic hydrogen charging (with respect to a sharp decreasing of their shear modulus, without a considerable change of their elastic modulus), on the basis of an analysis of the related experimental data, viz., a series of works on a sharp decrease of flow stress of iron mono- and polycrystals under electrolytic hydrogen charging (loading) (Bull. Jpn. Inst. Metal. 21 (1982) 26), works on anomalous plastic auto-deformation of iron under thermo-cycling around temperature of alpha double left right arrow gamma transformation in hydrogen atmosphere (the hydrogen superplasticity) (Phys. Met. Metallogr. 55 (1983) 805), works on "a deformation response" (as a sharp plastification) of solids (metals (Pd, V, Ta, Nb, Zr), amorphous Fe- and Co-based alloys, intermetallics (TiNi) and Laves phases) (Phys. Met. Metallogr. 81 (1996) 167; Phys. Met. Metallogr. 83 (1997) 139), works on the hydrogen amorphization of Zr3Rh type compounds (Yen et al.), works about high-pressure hydrogen influence on plasticity, strength and cyclic fatigue of steels (Workableness of steels and alloys in hydrogen environments, Livov, Ukraine, 1999), and also a plenty of the known works on the hydrogen plastification and superplasticity of titanium alloys (B.A. Kolachev and others), and particularly, TiAl intermetallics (Phys. Chem. Mater. Treat. 5 (1997) 101). (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.