An x-ray diffraction study carried out on the pentylenediammonium pentachlorobismuthate (III) compound [NH3(CH2)5NH3]BiCl5 indicated that it is orthorhombic with a P212121(Z = 4) space group. Differential scanning calorimetry (DSC), differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were also made, which showed that this compound undergoes a unique phase transition at high temperatures. The infrared and Raman spectra recorded at room temperature were interpreted on the basis of data published in the literature. The Raman bands observed below 300 cm−1 are very characteristic of the anions, which are made of isolated (BiCl52− )n units composed of BiCl63− octahedra. Raman spectra recorded at various temperatures confirmed the contribution to the phase-transition mechanism of both the anion and cation motions. Copyright © 2005 John Wiley & Sons, Ltd.