Irradiation of alkali silicate glasses results in the formation of metastable spin centers, such as oxygen hole centers (OHC 1 and OHC 2), silicon peroxy radicals and a silicon dangling bond (E ′ center). In this work, electron paramagnetic resonance (EPR) and electron nuclear double magnetic resonance (ENDOR) are used to study these spin-1/2 defects. It is shown that in a subset of the OHC 1 centers the ≡SiO radical is strongly coupled to a single alkali cation. Thermally activated swinging motion of this cation causes asymmetric T 2 relaxation and changes electron spin echo envelop modulation (ESEEM) spectra. It is argued that trapping of the hole by non -bridging oxygen atoms does not result in the release of a compensating alkali cation. Rather, the O–Alk bond elongates and the whole structure relaxes. This view is supported by semi-empirical and ab initio calculations. The observed axial symmetry of the g-tensor for OHC 2 is the result of rapid tunneling of the electron between two degenerate sites with rate (0.2−50)×10 11 s −1 and activation energy ∼10 meV. This center is a hole trapped on a tetrahedral >SiO 2 2− unit or a planar –SiO 2 − unit. It is demonstrated that silicon peroxy radicals are not formed by charge trapping. Their generation is temperature-independent and occurs via the decay of self-trapped excitons. It seems likely that the same process yields silicon dangling bond centers.