The stoichiometry and stability of lanthanide complexes with tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA) have been investigated by potentiometric titration. X-ray crystallographic analysis of the monohydrate has confirmed identification of the THFTCA as the trans-cis-trans geometric isomer, having the 2,5 and 3,4 carboxylate groups on opposite sides of the THF ring. THFTCA·H 2O crystallizes in the centric space group P 1 with a = 7.594(2), b = 8.137(3), c = 9.404(4) A ̊ , α = 107.08(3), β = 101.48(3), γ = 101.30(2)° and D calc = 1.70 g cm −3 for Z = 2. A variety of protonated complexes having M:L stoichiometries of 1:1 and 1:2 are propoed to describe the results of titrations with lanthanum(III), neodymium(III), europium(III), dysprosium(III) and thulium(III) (at 25.0 ° C and I = 0.1 M). The 1:1 complexes increase in strength from La to Dy, but Dy complexes are of similar strength to Tm species (except at pH>7). Stepwise stability constants for the 1:2 complexes suggest a size selectivity for Nd, but the overall stability of the 1:2 lanthanide complexes demonstrates no trend with the cation size. The complexes are significantly stronger than the structurally analogous lanthanide complexes with oxydiacetic acid. The increased stability and size selectivity reflects the influence of the favorable preorientation of the cation binding groups in THFTCA.