Single-crystal X-ray data were collected in steps of 25 °C up to 200 °C on a natural sample of zemannite, Mg 0.5 [ZnFe 3+ (TeO 3 ) 3 ]·4.5H 2 O. Ab initio simulations were performed to determine the hydrogen-bonding network to get better insight into the mechanism of the dehydration process. At room temperature, zemannite is hexagonal with cell parameters a = 9.4301(12) Å, c = 7.6692(10) Å, V = 590.63(13) Å 3 . The structure was refined in the non-centrosymmetric space group P 6 3 , instead of P 6 3 /m. Ab initio simulations suggested an ordered Fe/Zn distribution at the octahedral sites that resulted in agreement with P 6 3 symmetry. The dehydration starts at 100 °C and proceeds with continuous loss of the non-Mg-bonded H 2 O up to 125 °C. At 150 °C, the interstitial water is lost leading to 3H 2 O pfu. The polyhedral framework does not undergo significant modifications. The unit-cell volume slightly decreases from 590.63(13) Å 3 at room temperature to 577(2) Å 3 at 200 °C.