The X-ray crystal structure of an anionic octacoordinate Eu 3+ complex of the formula K 12 H 8 [Eu 4 (EDTMP) 4 ]·45H 2 O, where EDTMP is the ethylenediaminetetra(methylenephosphonate) anion, hereinafter referred to as I, has been determined. The crystal consists of cyclic tetrameric complex anions, in which the surrounding of each Eu 3+ ion is composed of two nitrogen atoms and six oxygen atoms from phosphonate groups. One of the phosphonate groups in the ligand molecule is tridentate (O:O′,O″), thus giving rise to the formation of the tetramers. The compound was characterized by UV–Vis electronic spectroscopy. At room and liquid nitrogen temperatures the complex shows luminescence from both 5 D 0 and 5 D 1 states, the latter one, which is very rare in Eu 3+ compounds with organic ligands, is probably brought about by the saturation of the coordination sphere with the phosphonate groups.