EN
The K_3H(SO_4)_2 doped with Cu^{2+} ion was studied in detail by X-band CW EPR. Two kinds of Cu^{2+} complexes (magnetically and structurally nonequivalent) denoted as Cu^{2+}(I) and Cu^{2+}(II) were found. Spin-Hamiltonian parameters, direction cosines, and coordination of Cu^{2+} ion were determined at room temperature. The superhyperfine structure was observed below 250 K as a result of interacting of Cu^{2+} ion with four equivalent protons (I=1/2). Temperature studies of K_3H(SO_4)_2 crystal show that interbond motion as a precursor of superprotonic phase transition occurs at above≈250 K.