Abstract Spin-lattice relaxation rates of 35Cl in KCl and 81Br in KBr were determined between 200 °C and the melting temperature T m. With the addition of Cu + (10–1000 ppm), Ag + and Li + pronounced maxima of the relaxation rates were observed below T m. They are due to the dynamic quadrupole interaction caused by the diffusion of the foreign ions. From the diffusion contribution to the relaxation mean jump frequencies of the impurities were derived. They are considerably larger than those of the K +-ions (for Cu + at 500 °C by a factor of 1000). The activation energies for the Cu +-diffusion are 1.1 eV for KCl and 1.0 eV for KBr with preexponential factors of 10 15 and 5·10 14 s -1, respectively. From comparison with relaxation measurements in KCl and KBr doped with divalent cations, it is concluded that Cu +, Li + and Ag + are not incorporated in interstitial positions. The relaxation measurements with crystals containing Na + and Tl + showed that their jump frequencies remain much smaller than 10 8 s -1 at T m.