Characteristic of metal oxalate complexes, these compounds tend to be polymeric, hence their low solubility in water.

The dihydrate FeC2O4(H2O)x is a coordination polymer, consisting of chains of oxalate-bridged ferrous centers, each with two aquo ligands.

When heated to 120 °C, the dihydrate dehydrates, and the anhydrous ferrous oxalate decomposes near 190 °C.

[4] The products of thermal decomposition is a mixture of iron oxides and pyrophoric iron metal, as well as released carbon dioxide, carbon monoxide, and water.

[7][8] A related, although much more complex mineral is stepanovite, Na[Mg(H2O)6] [Fe3+(C2O4)3]·3H2O is an unusual example of a naturally-occurring ferrioxalate.