At elevated temperatures the crystals of complex acid salts KmHn(SO4)(m + n)/2 . xH 2 O (0 ≤ x ≤ 1, m > 3, n ≥ 1) exhibit an anomalous temperature behavior of the dielectric permittivity and conductivity similar to the behavior observed in pure KHSO4 at the structural phase transition or on melting. This unusual behavior is due to the formation of multiphase states at higher temperatures where phases with different chemical composition coexist. It is shown that K3H(SO4)2 undergoes a ferroelastic phase transition between the point groups 3m↔2/m with anomalously slow kinetics at 463 K while the multiphase state is formed near 480 K