Differential scanning calorimetry (DSC) and impedance spectroscopy (IS) were used to study the composite solid electrolytes (1-x)KH₂PO₄-xAl₂O₃ (x=0.0-0.5). Furthermore we grew single crys-tals of pure KDP. The DSC curve of single crystal KDP show three endothermic peaks at 185, 223 and 256 °C, respectively, on the first heating run. For polycrystalline KDP, the first peak some-times appear at 183 oC and it tends to disappear, the second one shifts down to 195 oC and the third one appears at 261 oC. For the composites, only two endothermic peaks are observed on heat-ing runs, at about 210 and 250 °C, respectively. The dc-conductivity data for all samples show an Arrehenius behavior with temperature but the values for the singles crystals and the composites (x=0.1 - 0.4) remains about one order of magnitude higher than those for the polycrystalline ones in the whole temperature range. Moreover, the Arrhenius conductivity plots for the single crystals and the composites (x=0.1 - 0.5) samples show a change of slope to higher values at about 190 oC. The dc-conductivity data vs. alumina concentration showed two conductivity maxima at about x=0.1 and x=0.3 at different temperatures. The results are discussed in terms of the approximation effect of Al₂O₃ dispersed particles on the proton behavior of KDP.