EN
The mixed crystals of NaNH_{4}SO_{4}·2H_{2}O-NaNH_{4}SeO_{4}·2H_{2}O were grown and their properties were investigated. Dielectric and differential thermal analysis studies allowed us to construct the phase diagram of the system with molar fraction, x, of NaNH_{4}SeO_{4}·2H_{2}O as a parameter. Nonlinear dependence of T_{c} versus composition was found.