We have performed electron spin resonance (ESR), nuclear magnetic resonance (NMR) and static magnetic susceptibility measurements on heavily irradiated NaCl–KBF4 single crystals in the temperature range 4.2 < T < 350 K. In these samples, up to about 10% of the NaCl molecules are transformed into extremely small metallic Na particles and Cl2 precipitates. At high temperatures a one-line ESR signal, i.e. common mode due to strong exchange interaction between conduction electrons and F-aggregate centres, is observed. We propose that the smooth decrease of the ESR spin susceptibility with decreasing temperature, which can be as large as 50%, is due to a metal–insulator transition, taking place at about 40 K. In the same temperature range, the linewidth increases by 18±2 G with decreasing temperature. This anomalous broadening is explained by a reduction of the exchange narrowing at low temperatures. NMR spin–lattice relaxation on 23Na shows a Korringa-type behaviour down to 10 K, which suggests that the conducting phase in heavily irradiated NaCl–KBF4 behaves as a three-dimensional metal. SQUID experiments have revealed antiferromagnetic ordering at 40 K and a ferromagnetic phase below 20 K. The nature of the observed effects is discussed.