The presence of hydrogen (H) in steel may have a detrimental effect on its mechanical integrity, i.e. a ductility loss takes place. This phenomenon, often called hydrogen embrittlement, can cause unexpected failure. In this project, the effect of H in iron-based alloys with a specific microstructure will be studied. H can interact on a microscopic scale with defects in the materials, e.g. vacancies. Vacancies are empty spaces in the Fe lattice because an atom is missing. Plastic deformation, rapid cooling or irradiation can introduce a high concentration of these vacancies. They can gather and form complexes with interstitials such as hydrogen and carbon. These complexes will be present in most commercial steels and are very important for the nuclear sector. An innovative approach will be used to study these interactions. Two advanced techniques, positron annihilation and internal friction will be used complementary to the well-established methods for the evaluation of hydrogen/material interactions, such as thermal desorption and hot/melt extraction. Each technique allows studying one aspect; thermal desorption and hot/melt extraction are used to evaluate the role of hydrogen in the material, internal friction to study interstitial-vacancy clusters and positron annihilation is sensitive to vacancies. A full understanding of the interactions between H and vacancies will be obtained by this innovative combination of techniques.