Ba(H 2 O) 3 ](ClO 4 ) 2 between 90 and 300K possesses two solid phases. One phase transition of the fi rst-order type at: T h C = 211.3K (on heating) and T c C = 204.6K (on cooling) was determined by differential scanning calorimetry. The entropy change value ( Δ S

15 Jmol – 1 K – 1 ), associated with the observed phase transition, indicates a moderate degree of molecular dynamical disorder. Both, vibrational and reorientational motions of H 2 O ligands and ClO 4 – anions, in the high-temperature and low- temperature phases, were investigated by Fourier transform far-infrared and middle-infrared and Raman light scattering spectroscopies. The temperature dependences of the full-width at half-maximum values of the bands associated with r w (H 2 O) mode, in both infrared (~570cm – 1 ) and Raman light scattering (~535cm – 1 ) spectra, suggest that the observed phase transition is not associated with a sudden change of a speed of the H 2 O reorientational motions. Ligands reorient fast, with correlation time of the order of several picoseconds, with a mean activation energy value E a =5.1 kJ mol – 1 in both high and low temperature phases. On the other hand, measurements of temperature dependences of full-width at half-maximum values of the infrared band at ~460cm – 1 , associated with d d (OClO) E mode, and Raman band at ~1105cm – 1 , associated with n as (ClO) F 2 mode, revealed the existence of a fast ClO 4 – reorientation in phase I and in phase II, with the E a (I) and E a (II) values equal to 8.0 and 6.5 kJmol – 1 , respectively. These reorientational motions of ClO 4 – are slightly distorted at the T C .Fourier transform far-infrared and middle-infrared spectra with decreasing of temperature indicated characteristic changes at the vicinity of PT at T C , which suggested lowering of the crystal structure symmetry. All these experimental facts suggest that the discovered phase transition is associated with small change of H 2 O ligands and somewhat major change of ClO 4 – anions reorienta- tional dynamics, and with insigni fi cant change of the crystal structure, too.