The reversible bleaching kinetics of the F-band is investigated experimentally in several additively coloured alkali halides in that range of temperatures where F'-centres decay thermally. The results confirm the second-order recombination model discussed in an earlier publication. From the measurements these thermal ionization energies of the F'-centres are determined: KCl(0.44 f 0.02) eV; KBr (0.34 & 0.02) eV; NaCl(0.94 f 0.1) eV; RbCI (0.28 0.02) eV; RbBr (0.14 f 0.02) eV; RbI (0.24 f 0.04) eV. Furthermore, the recombination coefficients r, of the halogen-ion vacancies are estimated from the measurements provided that the recombination directly into the ground state dominates the recombination via the excited state F* with following emission of photons.