Dielectric measurements of Cs 2 SO 4 show a distinct relaxation at low frequencies at several isotherms (T < 651°C). For example, the relaxation frequency is around 5.1 kHz at 376°C and increases to approximately 1 MHz at 651°C. The relaxation has an activation energy of 0.97 eV, which is in close agreement whit that of Cs+ transport. We suggest that the observed dielectric relaxation could be produced by the Cs+ jump and SO 4 − reorientation that cause distortion and change the local lattice polarizability inducing dipoles like CsSO 4 −.