Various members of the FeSO 4 .H 2 O-CuSO 4 .H 2 O kieserite-type solid-solution series were synthesized and investigated by X-ray powder diffraction and Mossbauer spectroscopy. comparison of the Fe/Cu molar ratio between the precursor aqueous solutions and the products of precipitation revealed a preferred incorporation of Fe into the structure. End-member szomolnokite, FeSO 4 .H 2 O, is monoclinic, and synthetic CuSO 4 .H 2 O is triclinic. The cell-parameter refinements of (Fe, Cu)SO 4 .H 2 O compounds show a reduction in symmetry from monoclinic to triclinic beyond 20 mol. % Cu. By Mossbauer spectroscopy, significant ordering involving the cations can be observed only in members of the solid solution with more than 20 mol.% Cu. Site ordering is characterized by a preference of ferrous iron for the less distorted site. The similar scattering factors of Fe and Cu do not allow the cation distribution to be refined by X-ray methods. The structure of the investigated compounds with more than 20 mol.% Cu agrees with the triclinic model for CuSO 4 .H 2 O. Poitevinite is a distinct mineral species because of cation site-ordering and of the triclinic distortion of the structure. Its cell parameters, obtained by Rietveld refinement in space group P1, are: a 5.120(1), b 5.160(1), c 7.535(2) Aa, alpha 107.06(1), beta 107.40(1), gamma 92.73(1) degrees , and in space group C1: a 7.094(1), b 7.440(1), c 7.824(2) Aa, alpha 89.47(1), beta 119.56(1), gamma 90.45(1) degrees . The structural formula of poitevinite, as derived from ICP and Mossbauer data, is [Cu (sub 0.28) (Fe,Zn) (sub 0.22) ] [Cu (sub 0.19) (Fe,Zn) (sub 0.31) ]SO 4 .H 2 O.