Abstract-Along with their electromechanical coupling coefficients, the longitudinal dielectric, piezoelec tric and elastic characteristics in ferroelectric KH 2 PO 4 and antiferroelectric NH 4 H 2 PO 4 crystals are calcu lated using a modified proton ordering model that considers piezoelectric coupling and the four particle clus ter approximation. The possibility of detecting piezoactivity in solid solutions of K 1 -x (NH 4 ) x H 2 PO 4 is sub stantiated.