The structural and electronic properties of dimers, bulk, and alloys of Fe and V upon loading with hydrogen have been investigated using the density-functional method Spanish Initiative for Electronic Simulation of Thousand of Atoms. We have calculated the hydrogen-induced modifications of both the geometrical structure and the magnetic properties, which have been found to be closely related to each other. The general trends derived from our results are in good agreement with those found in the experimental characterizations of hydrogen-loaded Fe/V multilayer systems. In particular, we have found that hydrogen prefers a V environment when inserted in the alloy systems at high concentration; it occupies octahedral positions leading to a strong anisotropic expansion of the lattice accompanied by an increase in the local Fe magnetic moments. We have found that the main trends obtained in the extended systems were already present in the most stable geometrical and spin isomers of the freestanding FeVH clusters. © 2008 The American Physical Society.