Two new iron (III) phosphate oxalates have been isolated under hydrothermal conditions as phase-pure samples and their crystal structures determined from single crystal X-ray diffraction. [Fe₂(HPO₄)₂(C₂O₄)(H₂O)₂]·2H₂O, I, is a polymorph of a known phase and differs in the local arrangement of hydrophosphate, oxalate and coordinated water about iron, presenting a mer orientation of three coordinated phosphates, rather than fac as previously seen. The structure of I is three-dimensionally connected with similar network connectivity to the known phase but different overall topology. [Fe₂(HPO₄)₂(C₂O₄)(H₂O)₂], II, has a layered structure constructed from octahedral Fe(III) centres coordinated by hydrophosphate, oxalate and water in a fac arrangement. The amount of water used in synthesis is one of the key experimental parameters in stabilising one phase over the other. Thermogravimetric analysis shows that both I and II ultimately collapse into dense tridymite type FePO₄ above 600 °C and variable temperature powder XRD shows that this occurs via crystalline intermediate phases. Variable temperature magnetisation measurements show that both materials order antiferromagnetically at low temperatures, with similar Néel temperatures (∼29 K) despite their long-range structural differences.