The thermal dehydration of the fluoride hydrates FeAlF5 . 7H(2)O, Fe2F5 . 7H(2)O, FeVF5 . 7H(2)O and FeCrF5 . 7H(2)O has been investigated by thermoanalytical, X-ray and i.r. spectroscopic methods. The first dehydration step under quasi-isobaric conditions is characterized by the Formation of FeMF(5) . 2H(2)O phases. They are isotypic with Fe2F5 . 2H(2)O. The following dehydration proceeds differently. Only FeAlF5 has been found as definite dehydrated phase. Due to the second dehydration step, phases Fe2F5 . H2O, FeVF5 . 0.2H(2)O and FeCrF5 . 0.4H(2)O have been formed. Similar to FeAlF5 all these phases can be indicated orthorhombic. Obviously this orthorhombic structure has been stabilized by traces ofwater because the complete dehydration leads to the decomposition of this structure accompanied by the formation of FeF2 and the rhomboedrical MF(3)-phases. The compound FeAlF5 is stable up to 600 degrees C, then it decomposes by forming FeF2 and alpha-AlF3. Fe2F5 . H2O has been prepared as pure phase at the first time. Following from the lattice parameters structural relations exist between Fe2F5 . H2O, Fe2F5 . 2H(2)O and HTB-FeF3.