The chemical dehydration of Yb(ClO4)(3) . H2O by chlorine trioxide leads to a low-temperature form (LT) of anhydrous perchlorato complexes of ytterbium(III). By controlled thermolysis of this compound, a high-temperature form (HT) of Yb(ClO4)(3) was obtained. Its thermal decomposition leads to the corresponding oxide chloride, Yb3O4Cl, as a fine powder. The structures of the two polymorphic phases of Yb(ClO4)(3) were determined by Rietveld analysis of high-resolution X-ray powder data (BL16, ESRF). The LT form crystallizes in a hexagonal P6(3)/m unit cell (a = 9.219 70(7) Angstrom, b = 5.535 18(3) Angstrom, and Z = 2). For the HT form, the space group is R3c (a = 8.124 06(3) Angstrom, b = 24.0818(1) Angstrom, and Z = 6). The thermal decomposition pathway was also determined with the aid of TGA measurements and Raman spectroscopy.