In the present work, the results of an electron spin resonance ( ESR) study carried out on gamma irradiated solid piperacillin monohydrate ( PMH) were presented. While unirradiatedPMHexhibited no ESR signal, irradiated PMH was found to have an unresolved broad resonance line of singlet appearance. Results related with the variations of peak heights with applied microwave power, sample temperature, storage time at room temperature and annealing time at high temperatures were used to determine the structural and kinetic features and activation energies of the contributing radicalic species. Two tentative radicalic species were proposed and their spectroscopic parameters were determined through spectrum simulation calculations. The dosimetric potential of PMH was also investigated and it was concluded that PMH does not present the characteristics of a good dosimetric material, but that ESR spectroscopy could be used in the discrimination of irradiated PMH from unirradiated one.