Four new polymetallic iron(III) phosphonate cages have been made and structurally characterised. These are an octanuclear cage [Fe8O 3(OH)2(O2CtBu) 11(PhCH2PO3)3(py)3], a decanuclear cage [Fe10O2(OH)8(O 2CtBu)10(PhCH2PO3) 4(pip)2], a heterometallic cage [Fe6Li 5(μ3-O)2(tBuPO3) 6(O2CtBu)8(MeOH)2(Py) 4] and a tridecanuclear cage [Et3NH]2[Fe 13(μ3-O)3(μ2-OH) 7(tBuPO3)7(Me3CCO 2)14(H2O)] (pip = piperidine, py = pyridine). Magnetic studies of the first three compounds show anti-ferromagnetic exchange between the iron(III) centers leading to diamagnetic ground states for the homometallic cages. For the heterometallic cage, the six Fe(III) centers are arranged in two triangles, and each triangle has an S = 1/2 spin ground state. © The Royal Society of Chemistry 2009.