The influence of Co addition in NaCl crystals is studied by means of dielectric and optical measurements. Depolarization currents (ionic thermo-conductivity) due to the presence of dipoles "cobalt impurity-cation vacancy" are detected, and the effect of isothermal annealing on the dipole concentration is studied. The precipitation process which is observed has a third order kinetics, according to the results on the alkaline earth impurities. Optical absorption bands due to Co additions are detected. The presence of Co impurities catalyzes the formation of F centers, just as the presence of alkaline earth impurities, when the sample is X-irradiated. Nothing analogous to the Z bands is found