Detailed ESR investigation on irradiated black and rooibos tea was carried out in the dose range of 0.5-10 kGy. Unirradiated black and rooibos tea samples exhibit a weak, symmetric ESR (electron spin resonance) singlet centered at g = 2.0043 +/- 0.0010 with peak-to-peak line widths (Delta H-pp) of 1.00 +/- 0.05 and 0.64 +/- 0.05 mT, respectively. Irradiation caused a significant increase in ESR signal intensity of both samples without any changes in spectral patterns and these increases were found to be explained by a quadratic and/or an exponentially varying functions. Variation of ESR signal intensity, for the irradiated samples, at room temperature (295 K) with time in a storage period of 39 days showed that free radical responsible from the ESR spectrum of black tea is more stable than that of the rooibos tea. However, variable temperature and annealing studies show that the free radical responsible from the ESR spectrum of rooibos tea, with activation energy of 46.0 +/- 3.5 kJ mol(-1), is more resistant to the temperature than that of the black tea with activation energy of 33.8 +/- 3.10 mol(-1). (C) 2007 Elsevier Ltd. All rights reserved.