Single crystals of NaCl were subjected to about 5×105 roentgens of Co60 gamma irradiation at 5°C, and then rapidly heated and maintained at a fixed temperature in the range 65°C to 135°C. The dc ionic conductivity at this temperature was measured by means of a vibrating reed electrometer as a function of time. It decreased in the temperature range 65°C to 80°C during a period of several hours to a value that was less than the preirradiation conductivity by a factor which was typically about 30. The conductivity is presumed to be a direct measure of the number of positive ion vacancies. The data fit a rate equation describing the bimolecular process A+B-->AB, with an activation energy approximately equal to that of the mobility of the positive ion vacancies. This confirms the supposition that the change of conductivity reflects the clustering of the positive ion vacancies with other imperfections at a rate controlled by the diffusion of these vacancies. Above 100°C, many of the clusters appear to break up again with an accompanying increase of conductivity. The conducting vacancies appear to be localized in a rather small fraction of the total volume of the crystal.