Copper vinylphosphonate monohydrate, Cu(O3PC2H3).H2O, and zinc vinylphosphonate monohydrate, Zn(O3PC2H3).H2O, materials have been prepared using vinylphosphonic acid, and copper and zinc oxide, respectively, as precursors. Their crystal structures have been solved by single crystal X-ray diffraction and show that in copper vinylphosphonate, the Cu atoms are five-coordinate, whereas in zinc vinylphosphonate, the Zn atoms are in a distorted octahedral environment. In both structures, closest neighbour vinyl groups are separated by short contacts of ca. 4.15-4.3 Angstrom. However, due to the different coordination environments of the metal centres, the orientation of the vinyl groups in these two structures is not the same, which is expected to lead to different solid state reactivity with respect to the possibility of dimerisation or polymerisation reactions.