In a previous work, significant varistor behaviour was found in polycrystalline ZnO with Mn as the only additive. In the present work, the admittance spectra of these Mn-doped ZnO samples were investigated in the temperature range from 70 to 300 K. From this study, the microscopic parameters of the deep bulk donors such as their energy levels and capture cross-sections were obtained in the samples with different Mn contents. The origin of the donors in ZnO is discussed by comparing their characteristics found in Mn-doped ZnO with those observed in other ZnO varistor systems. (C) 2002 Elsevier Science B.V. All rights reserved.