In the present work, the electron spin resonance (ESR) dosimetric properties of naproxen sodium (NS) was investigated in the dose range of (2.5-25 kGy). Irradiated NS exhibited a very simple ESR spectrum consisting of a broadened antisymmetric single resonance line not saturating up to 2 mW microwave power at room temperature. The sum of two exponential increasing functions associated with two different radicals of different spectroscopic features and relative weights Acre found best describing experimental dose-response curve. Radiation induced radicals were observed to be very stable at room temperature but the increase in storage temperature increased very appreciably the decay of the contributing radicals, The results of the simulation calculation based on a model of two radicals showed that two carbon dioxide ionic free radicals ((CO2)-C-.)(-) of different orientational and environmental features produced by preferential rupture of carboxyl group from the rest of NS molecules in the crystalline matrix, were. likely at the origin of the experimentally observed ESR spectrum. Features such as good time stability of the signal intensity and relatively high radiation yield (G = 0.13) were considered providing NS with potential use as dosimetric material in measuring radiation dose in the range of 2.5-25 kGy by ESR technique. (C) 2003 Elsevier Science B.V. All rights reserved.