Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and impedance spectroscopy (IS) were used to examine the high-temperaure phase equilibria of KH2PO4 (KDP). The tested samples (named as KDP*) were prepared by heating KDP crystals up to 260°C. On subsequent heating the solid samples from room temperature, it was apparent thermal dehydration reactions of KDP* into polyphosphates, KnH2PnO3n+1(n>1). If the sample is heated above 300°C, such that all the constitutional water is released as a consequence of the dehydration reaction, then, a single solid phase (i.e. Kurrol’s salt C, KnH2PnO3n+1(n>>1)≅(KPO3)n is present in it, which is stable up to 800°C. An analysis of the frequency dependence of the electrical conductivity showed different effects of correlation on ion jumping in the various phases of KDP*. In particular, different mechanisms were infered from the temperaure variation of the “stretching” exponemtial parameter, β, and the cossover frequency to the dispersive region of the ionic conductivity, ωp, indicating a stable solid phase of KDP* below 260°C and a highly viscous phase above this transition temperature. The polyphospate phase or the inorganic polymer shows high proton conductivity whose optimization may find application in midto hightemepertaure use in the fuel cell emerging technology.