The diffusion of fluorine in KSn2F5 has been studied by T1 and T2 relaxation time measurements of l9F NMR (200−500 K) and pulsed magnetic field gradient techniques (390−480 K). Near 423 K a sharp transition into the superionic state has been found, the fluorine diffusion increasing by a factor of 4 within a range of 3 K. Conductivity measurements only show a change in the activation energy.