Gamma-irradiation of KF. 2H2O and KF. 4H2O crystalline hydrates at low temperatures results in the appearance of colour centres. The characteristics of optical spectra of these centres at 77 °K are: λmax= 475 nm andW1/2= 0.58 eV for KF. 2H2O; λmax= 480–485 nm and W1/2= 0.62 eV for KF. 4H2O. The EPR spectrum of colour centres is an isotropic singlet withg-factor 2.001 andΔH= 34.5 Oe (for KF. 2H2O at 77 °K). Their yields are low (0.015 and 0.0055 particles/100 eV for KF. 2H2O and KF. 4H2O at 77 °K, respectively). The kinetics of their accumulation at γ-irradiation as well as the kinetics of thermal and photostimulated decay has been investigated. On the basis of the data obtained it has been concluded that the colour centres in crystalline hydrates investigated are similar in their nature toF-centres in alkali halide crystals but differ from trapped electrons in glassy aqueous solutions.