The production by x-rays of anion vacancies in KBr crystals was studied quantitatively in the liquid helium temperature range. The thermal stability of the vacancies was investigated in detail. A series of recombination steps was observed in the 10°K to 30°K region associated with activation energies of the order of 0.03 to 0.09 ev. A green luminescence was observed during the irradiation by x-rays and during the annealing of the crystals. This luminescence is attributed to electronic recombinations rather than to vacancy recombination. A semiquantitative electrostatic model is proposed to explain the very limited thermal stability of single vacancies. This analysis predicts remarkably high local concentrations of vacancies.