Magnesium (Mg) and magnesium nickel (Mg-23.5 wt%Ni) reinforced with multiwalled carbon nanotubes (CNTs) were processed by powder metallurgy and then charged with hydrogen by annealing at 620 K under a pressure of 0.4 MPa of hydrogen. Mechanical spectroscopy revealed that such a treatment has no effect in the composites with pure Mg matrix. On the other hand, Mg-23.5 wt%Ni alloys, unreinforced as well as reinforced with CNTs, exhibit a mechanical loss spectrum, which is deeply modified by hydrogen charging. Two relaxation peaks are observed at similar to 190 K and similar to 330 K (frequency similar to 6 kHz), the height of which increases strongly with hydrogen charging. These peaks may be interpreted by atomic relaxation due to hydrogen in the Mg2Ni phase. Both peaks decrease strongly in height by degassing under vacuum at 750 K. In bulk specimens of Mg-23.5 wt%Ni/CNTs composites a concentration of hydrogen as high as 6.1 wt% (similar to 6% of specific weight of liquid hydrogen) has been obtained by the mentioned hydrogenation process. (C) 2009 Elsevier B.V. All rights reserved.