Characteristic features of the radiolytical intermediates produced in gamma irradiated solid sulfanilamide (SA) were investigated in the present work using ESR spectroscopy. SO2, which is the most sensitive group to radiation of SA molecule, was found to be at the origin of radiation produced ionic radical species. The latters give rise to an axially symmetric and an isotropic ESR spectra so that their sum appears as a three line antisymmetric ESR spectrum. Heights of these lines measured with respect to the base line were used to monitor microwave, temperature, time-dependent and kinetic features of the radical species contributing to ESR spectrum. Based on the experimental results derived from this study, it was concluded that as in the case of other solid sulfonamides radiation, yield of solid SA is very low (G = 0.5) compared with those obtained for sulfonamide aqueous solutions (G = 3.5-5.1), so that SA and SA-containing drugs could be safely sterilized by radiation. (C)2003 Elsevier B.V. All rights reserved.