Positron lifetime measurements revealed that huge numbers of excess vacancies were formed in addition to dislocations during the first H absorption process of LaNi5, and that excess vacancies became mobile and formed micro-voids via a thermal activation process. Theoretical calculations were made here, of positron lifetimes in the LaNi5–H system, by using first-principles electronic structure calculations. By comparing theoretical and experimental positron lifetimes, one of the defect components present during H absorption could be attributed to annihilation at vacancy-clusters composed of 2 or 3 Ni vacancies. The positron lifetime of the vacancy-cluster component increased over 400ps during isochronal annealing after H desorption. The vacancy-cluster could contain not only Ni vacancies but also La vacancies, since a vacancy-cluster composed of only Ni vacancies could not yield such a long positron lifetime.Theoretical Calculation of Positron Lifetimes for LaNi5–H System. M.Mizuno, K.Sakaki, H.Araki, Y.Shirai: Journal of Alloys and Compounds, 2003, 356-357, 186-90