Monoclinic FeSO4·7H2O is the stable solid phase between -1.82° and 56.6° C in contact with a saturated water solution of FeSO4. It occurs in nature as an oxidation product of Fe-containing sulfides an is called melanterite. FeSO4·7H2O belongs to a series of compounds Me2+SO4·nH2O, where Mn2+ is a cation with an approximate ionic radius of 0.7Å. The 1-, 4- and 5-hydrates are known to crystalize each in only one form, whereas the hexa- and the heptahydrates occur both in two different forms. The crystal structure of the tetragonal NiSO4·6H2O; Zalkin, Ruben and Templeton reported the structure of the monoclinic CoSO·6H2O. Of the structure of the heptahydrates but one was described: the orthorhombic form of NiSO4·7H2O. No details were known about one of the monoclinic heptahydrates, though Leonhardt and Ness published the cell constants and the space group of FeSO4·7H2O.. In addition they stated essentially correct positional parameters for the sulfur atom and gave the correct positions of the iron atoms. The present investigation has been undertaken as part of an extensive study of salt hydrates. A preliminary account has been published before.