The temperature dependence of the piezoelectric strain constantd14was investigated in mixed crystals isomorphous with Rochelle salt. Crystals oriented for utilization of Voigt's theory of the converse piezoelectric effect in the hemihedral class of the orthorhombic system were grown at constant temperature from supersaturated solutions of Rochelle salt containing isomorphous impurities C4H4O6Na NH4·4H2O or C4H4O6Na Tl·4H2O and measurements of the strain resulting from an applied electric field were made in the range 10 to 35°C. Hysteresis and saturation occur in the range of large piezoelectric activity whered14evaluated from the straight portion of the strain curve is approximately 5 × 10−4in both the pure salt and mixed crystal, a value several times as large as that reported from measurements of the direct effect in Rochelle salt. The critical temperature for a mixed crystal lies below that of pure Rochelle salt (25+273°K) and the shift observed compares favorably in magnitude with the value calculated from the dipole theory of dielectrics. The region in whichd14decreases to about 10−5covers 3°C. The value ofd14diminishes further as the temperature is raised.d14is defined as twice the change in length per cm length of a narrow crystal cut at 45° with theYandZaxes when unit electrostatic field is applied in theXdirection.