Unirradiated ammonium tartrate (AT) samples do not exhibit any ESR signal. However, irradiation produces an unresolved singlet at g = 2.0034 +/- 0.0006 with two shoulders at g(1) = 2.0093 +/- 0.0006 and g(2) = 2.0048 +/- 0.0006. The dose-response curve was found to increase linearly with the applied radiation doses in the range of 0.1-2.0 kGy and the slope of this curve was increased as the modulation amplitude increased. The activation energy value E(a) = 69.0 +/- 1.2 kJ/mol was calculated from Arrhenius plot for the radical species responsible from ESR spectrum of irradiated AT. (C) 2009 Elsevier Ltd. All rights reserved.