A novel hydrated fluoroselenate NaCoSeO4F 3 2H2O has been synthesized, and its structure determined. Like its sulfate homologue, NaCoSO4F 3 2H2O, the structure contains one-dimensional chains of corner-sharing MO4F2 octahedra linked together through F atoms sitting in a trans configuration with respect to each other. The magnetic properties of the two phases have been investigated using powder neutron diffraction and susceptibility measurements which indicate antiferromagnetic ordering along the length of the chains and result in a G-type antiferromagnetic ground state. Both compounds exhibit a N eel temperature near 4 K, and undergo a field-induced magnetic phase transition in fields greater than 3 kOe.