The complex anion obtained by crystallisation from an aqueous solution of molybdate and citric acid (H₃cit) at pH ca. 3 is of the type (i), [(MoO₂)₄O₃(cit)₂]⁴⁻, as determined by X-ray analysis of the salt K₄[Mo₄O₁₁(cit)₂]·6H₂O. Principal dimensions [ranges or means; estimated standard deviation (e.s.d.) 0.005 Å] are: MoO 1.692–1.734; Mo–O(bridging) 1.886–1.948; Mo–O(hydroxy) 1.972(5); Mo–O(carboxylate) (unidentate) 2.185–2.318, (bridging) 2.296–2.333 Å. The crystals are triclinic, space group P, a = 8.966(4), b = 12.319(6), c = 15.962(9) Å, α = 89.34(4), β = 99.80(4), γ = 107.04(4)°, R = 0.041 for 4 399 observed [I/σ(I) 2.0] reflections. The complex is tetranuclear with the citrato-ligands co-ordinated through the deprotonated hydroxy group and the α-carboxy group to form five-membered chelate rings. One of the oxygens of the β-carboxylate groups bridges between two Mo atoms [Mo(1)–Mo(2) and Mo(3)–Mo(4)] whereas the other oxygen bridges between these dimers. The other β-carboxylate group is not co-ordinated to molybdenum but is protonated. The i.r. and Raman solid-state spectra are in agreement with this structure as well as a study in solution (Raman, ¹H and ¹³C n.m.r. spectroscopy, molecular weight determination).