Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were used to study the high temperature phase behaviour of mixed crystal of K1-chi(NH4)(chi) H2PO4 in the KDP rich region (chi < 0.02). All of the mixed crystals show phase-transition like phenomena at a characteristic temperature T-p, few degrees above that for pure KDP, i.e. 180 degreesC. Our results show that T-p marks the onset of a thermal dehydration of the samples when they are heated above this temperature. The rate of the corresponding dehydration reaction is faster for samples with ADP impurities indicating that the density of reaction sites or their propagation from the surface to the interior of the crystal increase with the impurities.