Temperature dependences of the longitudinal permittivity, piezoelectric coefficient d 36, and elastic constant c 66 of K1 − x (NH4) x H2PO4 mixed crystals are studied experimentally. A microscopic model is proposed for crystals of the K1 − x (NH4) x H2PO4 type that includes the piezoelectric contribution to the effective pseudospin cluster Hamiltonian. Dielectric elastic and electromechanical properties of these crystals with ammonium concentrations x below 0.40 are calculated in a wide temperature range using the cluster approach. The calculation results are in qualitative agreement with experimental data.