Earlier independent crystal-structure studies of kieserite, Mg(SO4)·H2O, and the isotypic transition-metal compounds Me2+(SO4)·H2O (Me2+ = Mn, Fe, Co, Ni, Zn) indicated that the lattice parameters of kieserite are significantly larger than expected from both the respective ionic radii and the octahedral Mg/Me–O bond lengths. This volume mismatch was especially noticeable in comparison to the respective (high-spin) Co2+ compound with its only slightly larger ionic radius than Mg. In order to study this possible discrepancy, we synthesized several members of the kieserite–cobaltkieserite solid solution series, Mg1−xCox(SO4)·H2O, and investigated the structures of ten representatives, including the end-members, using consistent single-crystal X-ray diffraction data. The present results confirm the supposed anomaly, showing a significant negative correlation of polyhedral vs. unit-cell volumes, which both change in total by ± 1.2 Å3 over the series (Vkies. = 356.0, VCo-kies. = 354.8 Å3; <Mg–O> = 2.078, <Co–O> = 2.097 Å). This effect is realized by folding and rotations within the polyhedral assemblage, apparently driven by angular O–Me–O and especially Me–O–S/Me changes. Apart from this oddity, the Mg1−xCox(SO4)·H2O solid solution can be described as ‘well-behaved’, i.e. no indications for miscibility gaps or cation ordering effects were found, and cell parameters and other structural data behave according to Vegard’s law in linear relation with the refined Mg1−xCox content. Explanations for the paradoxical volume behaviour of kieserite compared to cobaltkieserite (and other transition-metal kieserites) are probably related to the absence, respectively presence of partly filled 3d orbitals and/or to well-established differences in electronegativity. Literature data on bond critical point properties of forsterite and transition-metal olivines indicate that the bond critical point rc lies significantly closer to Mg as compared to the transition cations, which might be ascribed in part to the absence of d orbitals at the Mg atom. In turn, the respective mutual influence on the electron density at the oxygen atoms in case of Mg–O bonds could represent a relevant modifier for the second coordination sphere and thus contribute to the paradoxical crystal chemical behaviour observed for kieserite as compared to isotypic transition-metal compounds in general and to cobaltkieserite in particular.