Effect of ammonium dihydrogen phosphate admixture on phase transitions in nanostructured solid solutions (1 − x)KH2PO4–(x)(NH4)H2PO4 at x = 0, 0.05 and 0.15 has been studied by dielectric spectroscopy. The samples have been prepared by embedding of aqueous solutions into porous borosilicate glasses. The X-ray diffraction have shown that the crystal structure at room temperature corresponds to the bulk KDP and the average nanoparticle diameters are 49 (2) nm for the sample with 5 % of (NH4)H2PO4 (ADP) and 46 (2) nm for the nanocomposites with 15 % of ADP. Dielectric response data analysis have revealed the shifts of the ferroelectric phase transition temperature as a function of (NH4)H2PO4 concentration: at x = 0 ∆TC is equal to ∼6 K, at x = 0.05 ∆TC ∼3 K and at x = 0.15 ∆TC ∼2 K.