The results show variation in phase boundaries and ionic conductivity of KH2PO4 (KDP) with the type of added impurities and temperature. At higher temperatures, irregular endothermic peaks are observed which are associated with the thermal decomposition of the samples whose onset temperature depends on the sample. Electrical characterization shows enhancement of ionic conductivity in single crystal samples either doped or radiated with respect to that of pure KDP, either polycrystalline or single crystal samples. Our results thus show that structural phase transition occurs before the thermal dehydration of KDP only on single crystal samples and the onset of this reaction depends on the distribution of the point defects induced in the sample by doping or radiation. The results are explained in terms of the rigidity of the hydrogen bonds sub-lattice which is modified by these treatments. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)