We have implanted hydrogen (or deuterium) in magnesium single crystals and used the techniques of nuclear reactions analysis to measure the concentration profiles and the lattice location of the implanted hydrogen. For an implantation at very low temperature (40 K) hydrogen occupies tetrahedral sites. This configuration remains during annealing cycles up to 110 K where hydrogen becomes mobile and is trapped after a few jumps by vacancies. Hydrogen clusters are formed by annealing above 225 K and are subsequently dissolved at a temperature of about 300 K.