The mixed valency (M2+M3+) sulfate minerals, römerite Fe2+Fe23+ (SO4)4· 14H2O and botryogen Mg2+Fe3+ (SO4)2(OH)·7H2O have been studied using Raman spectroscopy. The Raman spectra of the two types of crystals proved very similar but not identical. The observation of two symmetric stretching modes confirmed the presence of the two nonequivalent sulfate units in the römerite structure. The observation of multiple bands in the antisymmetric stretching regions and in the bending regions proves that the symmetry of the sulfate anion is significantly reduced in the römerite structure. The number of Raman bands related to the (SO4)2− symmetric and antisymmetric vibrations supports the X-ray single crystal structure conclusion that two symmetrically distinct S6+ are present in the structure of botryogen. Römerite is a mineral of environmental significance as it is commonly found in tailings and dumps. Copyright © 2010 John Wiley & Sons, Ltd.