Abstract Positron annihilation in structural vacancies in B 2 Al‐rich NiAl alloys is studied theoretically. For this purpose we use the atomic superposition method combined with molecular dynamics employed for relaxations of random vacancy configurations. We found that the defect lifetime decreases with the increasing Ni vacancy concentration. Calculated positron lifetimes are discussed in terms of the ‘vacancy‐superstructure' effect and compared with experimental positron lifetime data available. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)