Fiber-optic Bragg-Grating sensors are often the only possibility to perform measurements under adverse environmental conditions, ionizing radiation among them. However, up to now no basic research has been carried out to determine the influence of ionizing radiation on the operativeness of Bragg-Grating-Sensors. Within this work gratings of type I have been written into 3 different singlemode-fibers, with different Ge-content, one of them hydrogen-loaded. Various kinds of ionizing radiation (electrons, photons, He-nuclei) of various energy have been applied to the gratings and grating-temperature-sensors with doses up to 90 MGy. Changees of grating-properties have been monitored before and after irradiation. Experiments show only slight increase of the Bragg-wavelength within the measurement-precision of 1E-03 nm. According to these changes which were also estimated by theoretical considerations, typical applications for grating-sensors with exposure to ionizing radiation have been examinded to tell whether the sensors will meet the specific demands of the respective application.