Single crystals of Cu-substituted bieberite (Co1–xCux)SO4·7 H2O have been synthesized from aqueous solution by the evaporation method. It is shown that the solubility of Cu2+ in bieberite is limited to 0.46 atoms per formula unit (apfu). Chalcantite CuSO4·5H2O only accommodates a very small amount of ~0.03 Co2+ atoms. Structure analysis by X-ray diffraction of single crystals along the join CoSO4·7H2O–CuSO4·5H2O showed that the hepta-hydrate phases are monoclinic with P21/c symmetry at room temperature, whereas the penta-hydrate phases are triclinic, P1. Within the Co1–xCuxSO4·7H2O series (0 ≤ x ≤ 0.46), lattice parameters are distinctly altered by the Cu2+ substitution, however, the observed changes cannot be ascribed to different ionic radii of Co2+ and Cu2+ but are due to an increasing distortion of the lattice by increasing Cu2+ content. Cu2+ is not distributed randomly over the two possible crystallographic metal sites but exclusively enters the M2 site. While all the oxygen atoms coordinating the M1 site act as donors of hydrogen bonding to the sulfate tetrahedron, one oxygen atom at the M2 site is a hydrogen bond acceptor. This may cause the slight tetragonal elongation of the M2 coordination polyhedron, which favors the preferential occupation of this site by Cu2+. Optical spectra show typical features of both the Co2+ and the Cu2+ absorption signatures. While in Co2+-rich hepta-hydrate phases Co2+ absorption bands arising from both M1 and M2 sites are detected, there are only bands arising from the M1 site in Co0.54Cu0.46SO4·7H2O. This supports the observation of selective substitution of Cu2+ at the elongated M2 octahedral site. Cu2+ absorption bands can be assigned on the basis of D4h symmetry with increasing splitting of the Cu2+, 2T2g, and 2Eg energy levels with increasing Cu2+ content.