The Curie point of KD2PO4is found to be 222°±1°K, which is 9°K higher than previously reported. It is believed that the higher value is due to more complete deuteration. Other physical properties (at room temperature unless otherwise noted) are as follows: dielectric constant from 208° to 300°K; contour shearmode resonance‐frequency constant from 270° to 320°K; elastic compliance,s66E=165±3×10−12m2/N; piezoelectric constant,d36=58±2×10−12C/N; electromechanical coupling,k36=0.22±0.01; optical absorption between 200 and 2600 m&mgr; for both KD2PO4and KH2PO4(the transmission of a 7.4‐mm‐thick KD2PO4sample was greater than 50 percent from 400 to 1850 m&mgr;); zero stress electro‐optic coefficient,r63=26.4±0.7×10−12m/V; lattice constants for both KD2PO4and KH2PO4; and spontaneous polarization and coercive field from 150° to 222°K.