A novel hydrated fluoroselenate NaCoSeO4F center dot 2H(2)O has been synthesized, and its structure determined. Like its sulfate homologue, NaCoSO4F center dot 2H(2)O, 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 Neel temperature near 4 K and undergo a field-induced magnetic phase transition in fields greater than 3 kOe.