In this work have been presented the result of investigation of K_0,945Ag_0,055NO₃ and K_0,945Cs_0,055NO₃ single crystals were grown from aqueous solution of KNO₃, AgNO₃ and CsNO₃ using isothermal crystallization method. Then, structural and phase transformations in samples were studied by X-ray and optical microscopy methods. It has been determined that in K_0,945Ag_0,055NO₃monocrystal at T>393K temperature, in K_0,945Cs_0,055NO₃ monokrystal at T>455K temperature the rhombic second to hexagonal third transformation occurs with the formation and growth of the third modification crystal embryo within secondmodification. The results obtained from the kinetic studies were determined that the temperature dependence of the conversion rate of second to third in K_0,945Ag_0,055NO₃ and K_0,945Cs_0,055NO₃ single crystals can be expressed by an empirical formula is temperature delay, is transformation temperature and is equilibrium temperature between interconverting modification crystals. Based on the results of velocity measurements, the activation energy of polymorphic transformations were calculated.