Positron lifetime spectroscopy is a sensitive tool for the study of vacancy-type defects in solids. Our positron lifetime measurements have revealed that huge numbers of excess vacancies are formed in addition to dislocations during the first hydrogen absorption process in some hydrogen storage materials. In this work, we have performed first-principles calculations of positron lifetimes of vacancies as well as the bulk state in LaNi5-H and Pd-H systems in order to identify lattice defects formed during hydrogen absorption and during isochronal annealing after hydrogen desorption. By comparison between the experimental and theoretical positron lifetimes, one of the defect components formed during hydrogen absorption can be ascribed to the annihilation at the vacancy clusters composed of two or three vacancies in both LaNi5-H and Pd-H systems. During isochronal annealing after hydrogen desorption in LaNi5, the positron lifetime of the vacancy-cluster component in LaNi5 increases from 190 to over 400 ps, which indicates the formation of three-dimensional vacancy clusters. On the other hand, the vacancy-cluster component in I'd decreases from 200 to 160 ps during isochronal annealing. This suggests that not three-dimensional but two-dimensional vacancy clusters are formed in the recovery process of vacancies in Pd. (C) 2004 Elsevier B.V. All rights reserved.