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Electron-irradiation-induced amorphization in coesite was observed in situ as a function of temperature ͑15-750 K͒ at an incident electron energy of 1.0 MeV by transmission electron microscopy. Amorphization induced by ion-beam irradiation ͑1.5 MeV Kr ϩ ͒ in coesite was studied in situ as a function of temperature ͑15-875 K͒ using the high-voltage electron microscope ͑HVEM͒ Tandem Facility at Argonne National Laboratory. Electron-irradiation-induced amorphization in coesite was also observed at 200 keV and 300 K. Previously, the effect of temperature on amorphization was considered to only result in annealing, that is recovery of the damaged region. In this study, we show that temperature has an enhancing effect on amorphization and may even play a dominant role in the radiation-induced amorphization of some crystalline materials. The effect of temperature in enhancing amorphization was first theoretically and experimentally recognized in electronand ion-beam-irradiation-induced amorphization of coesite. Coesite has a melting temperature, T m ͑875 K͒, below its glass transition temperature, T g ͑1480 K͒. A thermodynamic analysis has been made to model the critical amorphization dose-temperature dependencies of electron-and ion-beam irradiations. We propose that the thermodynamic contributions to amorphization include the free-energy increase due to defect accumulation caused by irradiation and chemical disordering, ⌬G def and ⌬G dis , and a thermal contribution, ⌬G therm . The thermodynamic condition for amorphization is generalized by the expression ⌬G total ϭ⌬G therm ϩ⌬G def ϩ⌬G dis у⌬G am , where ⌬G total is the total free-energy increase for the irradiated crystal, and ⌬G am is the free-energy level required for solid-state amorphization. Coesite provides a good example of the dominance of the thermal contribution in amorphization. Solid-state amorphization of coesite induced by electron and ion irradiations is an irradiation-enhanced thermodynamic melting process below the glass transition temperature, T g . The competition among thermally created dynamic displacements, irradiation-induced static displacements, and the increased annealing rate at elevated temperatures, determines the critical amorphization dosetemperature dependencies of electron-and ion-irradiated coesite.