Guided tissue regeneration (GTR) and guided bone regeneration (GBR) biomaterials have been employed in recent years for periodontal procedures. In the present study, widely used dental GTR/GBR biomaterials (grafts: G1, G2, G3 and membranes: M1, M2, M3, M4) were exposed to gamma irradiation at an absorbed dose range of 0-50 kGy and the radiolytic intermediates that have been created in the samples upon irradiation were characterized in detail by Electron Spin Resonance (ESR) spectroscopy. We aimed to standardize the measurement conditions for practical applications of gamma radiation sterilization of GTR/GBR biomaterials. We investigated the characteristic features of free radicals in gamma irradiated GTR/GBR biomaterials and examined the stability of the induced radicals at room temperature and accelerated stability conditions with ESR spectroscopy including dose-response curves, microwave power studies, dosimetric features of the biomaterials, variations of the peak heights with temperature, and long term stabilities of the radical species. Long-term stability studies have shown that G1 is quite stable even in accelerated storage conditions. The signal intensities of graft-type GTR/GBR biomaterials stored in normal and stability conditions have decreased very rapidly even only a few days after gamma irradiation sterilization. Thus, those samples indicating relatively low stability features can be very good candidates for the radiosterilization process. The beta-tricalcium phosphate and PLGA containing G1 and M1 respectively have found to be the most gamma stable bone substitute biomaterials and be safely sterilized by gamma radiation. ESR spectroscopy is an appropriate technique in giving important detailed spectroscopic findings in the gamma radiation sterilization studies of GTR/GBR biomaterials.