A new Co(II) phosphonate, Co(H2O)2PO3C–C12H9·H2O, has been synthesized under hydrothermal conditions. The monoclinic P21/c structure of this organic–inorganic hybrid consists of isolated perovskite-type chains of corner-shared CoO4(H2O)2 octahedra interconnected via phosphonate groups. The unique one-dimensional structure of this phase is closely related to the single-chain magnet (SCM) phosphonate Co(H2L)(H2O), with L = 4-Me-C6H4-CH2N(CPO3H2)2, that contains isolated chains of CoO5N octahedra. Like the latter, this hybrid exhibits 1D antiferromagnetic interactions and the possibility of an effective pseudo spin contribution due to spin canting at low temperature, but, in contrast, is not an SCM. This different magnetic behavior is explained by the different geometry of the octahedral chains and by the possible existence of weak antiferromagnetic interactions between the chains. This opens the route to the investigation of a large series of compounds by tuning the chemical composition and structure of the phosphonic acid used as organic precursor of hybrid materials.