The crystal structure of CoHPO 3 , H 2 O, CP, has been determined. The structure consists of chains of phosphite-bridged Co 2+ ions extended along the a axis of the orthorhombic cell [space group Pca2 1 , a=8.984(2), b=7.918(2), c=10.139(8) A, Z=4]. The [CoO 6 ] octahedra form pairs successively tilted with respect to each other along the chain. The low temperature differential magnetic susceptibility shows that the material attains 3d antiferromagnetic order at 10 K. The magnetic absorption peak at this critical temperature indicates the presence of weak ferromagnetism, and magnetization experiments show a metamagnetic transition