Possible mechanisms of aggregation of donor nitrogen (C center) into a pair of nitrogen atoms (A center) in Ni-containing synthetic diamonds with NV centers subjected to high temperature annealing without external pressure are discussed. It was shown that the activation energy of aggregation varied from 0.5 to 1.5 eV depending on nitrogen concentration. This can be due to three different aggregation mechanisms taking place at the same time. The formation of isolated vacancies and nitrogen interstitials was studied in electron-irradiated diamonds containing different concentrations of donor nitrogen. (C) 2010 Elsevier B.V. All rights reserved.