The title compound β-phenylethylammonium pentachlorothallate III crystallizes in the monoclinic system with space group P2 1. The unit cell dimensions are: a=9.5470(1) Å, b=7.5660(1) Å, c=15.9180(1) Å, β=101.3420°(9), with Z=4. Its crystal structure was determined and refined down to R=0.0402. The structure of this compound consists of β-phenylethylammonium cations and polynuclear anions in which slightly distorted [TlCl 6] 3− octahedral sharing two cis vertices are interconnected into chains. These chains are themselves interconnected by means of the NH…Cl hydrogen bonds. Two phase transitions at T 1=353 K and T 2=389 K were detected and studied by differential scanning calorimetry, dielectric and impedance spectroscopy measurements. The evolution of the dielectric constant and dissipation factor as a function of frequency and temperature revealed the first transition to be ferroelectric–paraelectric. Crystallographic studies show that this material presents a noncentrosymmetric space group at room temperature which is in agreement with our last deduction. The second phase transition at 389 K is characterized by a strong jump in the conductivity plot. It is assumed that the reorientation of NH 3 + ions is responsible for this phase transition.