Abstract The oscillator strength of the F-centre has been chemically determined as f = 0.93 ± 0.11. From the transformation of F- into Z 1- and Z 2-centres, the values of f of the latter were estimated to be: 0.9 ± 0.2. The ratio of the electron capture cross-sections in the formation of Z 1- and F-centres respectively was found to be 0.15. Under irradiation with visible light, a stationary state F⇄ Z 1 is established, which depends on the wave-length of the light. From the thermal decomposition of Z 1- into F- and Z 2- centres, which has an activation energy of 1.7 eV, it was concluded that Z 1- and Z 2-centres are formed from the same type of Sr centres. The dichroism Z 1– Z 4 was found to be analogous to the F-M dichroism. These results are explained by a scheme where a Z 1-centre is a combination of a F-centre with a complex of a Sr-atom and a F-vacancy, and where a Z 2-centre is represented as a Z 1-centre which has lost a cation-anion vacancy pair. The mobility of the anion vacancy left by optical ionization of a F-centre is attributed to the gain in polarization energy due to the relaxation of the lattice.