Abstract The dispersion curves of the dielectric response of KH 2 PO 4 were obtained from 5 Hz to 13 MHz and over the temperature range 120–206 °C for both single crystal and polycrystalline samples. Both types of samples reveal dielectric relaxation at low frequency, for example around 10 4 Hz at 140 °C, which shifts to higher frequencies (∼1 MHz) as the temperature increases. The relaxation frequency was determined from the peak obtained in the imaginary part of the permittivity as well as from the derivative of the real part of the permittivity. The activation energy E a = 0.82 eV, obtained from the relaxation frequency is very close to that derived from the dc conductivity. We suggest that this dielectric relaxation could be due to the proton jump and phosphate reorientation that causes distortion and change the local lattice polarizability inducing dipoles like . (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)