A new organic–inorganic hybrid cobalt(II)–molybdenum(V) phosphate polymer incorporating piperazine (pip), (H 2 pip) 3 [Co 3 Mo 12 O 24 (OH) 6 (PO 4 ) 8 (H 1.5 pip) 4 ]·5(H 2 O), was prepared under hydrothermal conditions. As revealed by single-crystal X-ray diffraction studies, the material is modular, built from a secondary building block composed of two anionic hexameric polyoxomolybdophosphate [Mo 6 O 12 (OH) 3 (PO 4 ) 4 ] 9− moieties, bridged by a central octahedral Co 2+ centre. The sandwich-type {Co[Mo 6 O 12 (OH) 3 (PO 4 ) 4 ] 2 } 16− dimers are connected via tetrahedral Co 2+ metal centres, forming an infinite one-dimensional polymer. The compound constitutes the first example of a reduced sandwich-type cobalt–molybdenum phosphate in which the organic moiety (pip) is effectively coordinated to the inorganic backbone of the polymer, in this case via the tetrahedrally coordinated Co 2+ centres. The magnetic behaviour of this material was investigated in the temperature range 4–298K.