In the present work, the kinetics of Mn2+ ion and radiation induced free radical in gamma irradiated soybean were investigated in detail using ESR spectroscopy in the temperature range of 130-400 K. Like other legumes, such as lentil, pea and broad bean, the EPR spectrum of untreated soybean is composed of an equally spaced sextet due to Mn2+ ion and a singlet resonance line both appearing at g = 2.00 +/- 0.0010. Radiation was observed to cause an exponential increase in the intensity of the singlet resonance line but not creating any effect on Mn2+ ion resonance line intensities in the studied dose range of 1.25-15 kGy. Reversible increases of Mn2+ and free radical signals were observed below room temperature. Above room temperature, they both decreased and these decreases were irreversible. The kinetic features of Mn2+ and free radical signal intensities were studied by annealing the samples at predetermined temperature for predetermined time over the temperature range of 308-373 K. Ground soybean samples were used throughout the experiment to avoid any possible problems arising from the microwave cavity filling factor. (C) 2003 Elsevier Ltd. All rights reserved.