The dielectric permittivity of polycrystalline KESO4 was measured from 5 Hz to 13 MHz over the temperature range 51-187 degrees C. The corrected imaginary part of permittivity, epsilon", and its real part epsilon' vs. frequency clearly show a new dielectric relaxation around f(max) = 1.5* 10(5) Hz at 51 degrees C which shifts to higher frequencies (similar to 1 MHz) as the temperatures increases. If the measurements are done when increasing the temperature, f(max) decreases sharply (more than two orders of magnitude) around 164 degrees C; however, above this transition temperature, it continues shifting to higher frequencies as the temperature increases. However, if the measurements are done when decreasing, the temperature, f(max) increases abruptly around 146 degrees C, thus showing a hysteretic behaviour. We suggest that the observed dielectric relaxation could be produced by proton jumps and SO4- reorientation, causing distortion and changing the local lattice polarizability to induce dipoles like HSO4-. (c) 2005 Elsevier B.V. All rights reserved.