One of the main challenges for introduction of a hydrogen-based economy is storage of hydrogen. Hydrogen storage in solid materials is considered among the most attractive methods. During recent years much emphasis has been placed on the synthesis of nanosized metals and alloys. In the present study Mg(BH4)(2) and Mg((11)BD(4))(2) are infiltrated in pre-treated activated carbon and investigated with small-angle neutron scattering (SANS). The infiltration method is shown to be successful in modifying the size of the Mg-borohydride particles, as confirmed by scanning electron microscopy and x-ray diffraction data. The size of the particles for the infiltrated samples is estimated by SANS measurements to be mainly in the range <4 nm. The results suggest that the smallest pores of the scaffold are partially or fully filled and that this type of scaffold acts as an effective dispersing agent for Mg-borohydride.