AbstractThe Brillouin scattering from shorted and free single crystals of CsH2AsO4 and CsD2AsO4 is studied as a function of temperature. Utilizing the (z + x)[y, (z + x)] (z − x) scattering geometry, the softening of the elastic constant C is observed on approaching the Curie temperature from above. The results, which are also compared with similar data on KH2PO4 and KD2PO4, indicate a strong piezoelectric coupling between the acoustic shear mode and the soft optical ferroelectric mode. The temperature of the Brillouin shifted frequencies for all the KDP isomorphous crystals can be computer fitted with a Curie‐Weiss law for the soft mode. The difference between the extrapolated temperature of the shorted and free crystals are also explained on the basis of the thermodynamic relations of the coupled modes and are thus similar to those obtained for the clamped and free dielectric responses.