Reaction of p-tert-butylcalix[8]arene (4,11,18,25,32,39,46,53-octa-tert-butyl[1_.8]metacyclophane-7,14,21,28,35,42,49,56-octol, H₈L) under carefully controlled conditions in dimethylformamide (dmf) solvent enables the isolation of 1:1 complexes with both europium(III) and calcium(II) ions. Crystal structure determinations have been performed at ≈295 K on [Ca(H₆L)(dmf)₄]·3dmf [monoclinic, space group P2₁/c(C_2h⁵, no. 14), a= 22.24(2), b= 25.57(2), c= 22.07(2)Å, β= 115.91(5)°, Z= 4, R= 0.13 for 4915 ‘observed’ reflections] and [Eu(H₆L)(NO₃)(dmf)₄]·3dmf [monoclinic, space group C2/c(C_2h⁶, no. 15), a= 42.35(2), b= 18.48(2), c= 35.03(1)Å, β= 109.26(3)°, Z= 8, R= 0.086 for 7950 ‘observed’ reflections]. Both complexes involve the calixarene in a hitherto unobserved bidentate co-ordination mode, with the metal-ion co-ordination spheres being filled by attachment of O-bound dmf molecules and (in the case of europium) one bidentate nitrate ion, the calcium thereby being six- and the europium eight-co-ordinate. The double deprotonated p-tert-butylcalix[8]arene ligand adopts a near-planar conformation which, in the case of the calcium complex, is very similar to the ‘pleated loop’ form known for the free calixarene. Proton NMR spectroscopy suggests that in solution the europium complex is both less flexible and more stable than the calcium species.