Abstract The electrical conductivity measurements on single crystals of KH 2PO 4 and KH 2AsO 4 have been carried out in the temperature range from room temperature to 200 °C. The value of k B Tε″ showed a high-temperature anomaly around T p (182 °C for the KH 2PO 4 and 180 °C for the KH 2AsO 4). The total activation energies are obtained E t=0.42 in KH 2PO 4 and E t=0.40 in KH 2AsO 4 for the ionic conduction of mobile ions, including bulk and surface conduction. From complex ac impedance spectra, we observed the impedance consisting of two regions for measuring frequency: the high frequency region may be due to proton migration in the bulk, while the low frequency region may be due to charge transport at the surface. We separated bulk conductivity by means of complex impedance formalism. The bulk activation energies are obtained E B =0.68 eV in KH 2PO 4 and E B =0.59 eV in KH 2AsO 4.