07-18U

The effect of hydrostatic pressure on thermodynamic properties of Rs and RbHSO$_4$ was theoretically considered within the framework of the Mitsui model. In case with Rs, piezoelectric interaction with the shear strain $\varepsilon _4$ was taken into account. The approach, according to which theory model parameters are linearly dependent on hydrostatic pressure, was proposed for consideration. Theoretical results for thermodynamic characteristics were compared to experimental data. Basing on theoretical study, the explanation for discrepancies in experimental data derived by different researchers for static dielectric permittivity of a free crystal was proposed. The increase of saturation polarization due to application of hydrostatic pressure is predicted.