Single crystals of hydrated and anhydrous carnidazole have been subjected to X-irradiation at 77 K and studied by EPR. The determination of the EPR tensors shows that NO2, sulfur-centered radicals as well as a radical pair can be generated from this molecule; it appears that the species trapped at 77 K are not the same in the hydrated and in the anhydrous crystal. The results are discussed in the context of the radiosensitizing action of molecules containing a 5-nitroimidazole moiety.