Changes in Gunn diode performance caused by 1.25 MeV Co-60 gamma radiation have been studied. Hall effect measurements as a function of temperature made on GaAs crystals grown by liquid epitaxy reveal an energy level 0.13 eV below the conduction band which was not detectable before irradiation. Detailed measurements of low-field resistance were carried out on Gunn diodes together with output power and FM noise in a fixed X-band cavity as a function of gamma ray fluence. The power output was degraded by 3 dB after a fluence of about 3×1017photons/cm2and oscillations ceased at fluences two to three times this value. The FM noise increased only slightly and can be related to the change in matching of the diodes to the cavity. These effects are explained in terms of carrier removal, in that irradiation generates deep levels which trap conduction electrons and reduce the carrier concentration. A theoretically derived power degradation curve for Gunn diodes is given and compared with the experimental results.