Electron paramagnetic resonance (EPR) studies have been carried out on KU1 fused silica irradiated with neutrons at fluences 10 21 and 10 22 n/m 2 , and gamma-ray doses up to 12MGy. The effects of post-irradiation thermal annealing treatments, up to 850°C, have also been investigated. Paramagnetic oxygen-related defects (POR and NBOHC) and E′-type defects have been identified and their concentration has been measured as a function of neutron fluence, gamma dose and post-irradiation annealing temperature. It is found that neutrons at the highest fluence generate a much higher concentration of defects (mainly E′ and POR, both at concentrations about 5×10 18 spins/cm 3 ) than gamma irradiations at the highest dose (mainly E′ at a concentration about 4×10 17 spins/cm 3 ). Moreover, for gamma-irradiated samples a lower treatment temperature (about 400°C) is required to annihilate most of the observed defects than for neutron-irradiated ones (about 600°C).