Mechanochemical reaction of metal nitrides (Li3N, Ca3N2, h-BN, Mg3N2, Si3N4, AlN, TiN, VN, ZrN) was performed in a planetary ball mill. We demonstrated that hydrogen (H-2) can be stored by the ball milling of their respective metal nitrides in a H-2 atmosphere at 1 MPa and room temperature. The H2 content of ball-milled metal nitrides was 0.2-5.0 wt.%. This reaction did not occur at ambient conditions without a mechanochemical reaction mechanism. Infrared absorption showed that the N-H stretching vibrations of the ball-milled light metal nitrides occurred at 3100-3500 cm(-1). The N-H force constant of the metal imides in the ball-milled metal nitrides decreased with the metal ionic radius and the electronegativity. These results indicated that the size of the metal ion and the charge transfer from the metal to nitrogen play important roles for the force constant. (c) 2006 Elsevier B.V. All rights reserved.