EN
EPR of Mn^{2+} impurity ion in the (NH_{4})_{3}H(SO_{4})_{2} crystal is studied in the temperature range 80-420 K. It is shown that Mn^{2+} substitutes NH_{4}^{+}(I) ion and is coordinated in deformed oxygen octahedron in which takes two "off-center" positions. Its spin-Hamiltonian parameters and direction cosines of crystal field are determined. The successive EPR line splitting for three phase transitions is discussed.