An in situ infrared spectroscopic analysis of the thermal dehydration of zirconyl chloride octahydrate was carried out to identify bending mode vibrations of distinctive water molecules in this well-defined zirconium(IV) cluster cation. TG-MS analysis revealed the particular temperatures where one water molecule at a time was removed from the solid hydrate. In situ IR data unveiled remarkable spectral changes featuring isosbestic behavior. We were able to experimentally distinguish between delta(H2O) modes from coordinatively bound water molecules (Zr-(OH2)(3), 1595 cm(-1)), tetrahedrally coordinated lattice water (H2O dimer, 1620 cm(-1)), as well as strongly H-bonded lattice waters accommodating hydrated protons (1705 and 1666 cm(-1)). Spectral changes in the range from 1050 and 900 cm(-1) during dehydration are discussed.