Iron(II)-ethylene-bis-(phosphonates), Fe2[O3P-(CH2)2-PO3]·2H2O was synthesized by reaction of [FeSO4]·7H2O and the corresponding ethylene-bis-(phosphonic acid) and it was characterized by X-ray powder diffraction, thermogravimetry, UV-visible and infrared spectroscopy, and the magnetic properties were studied by a SQUID magnetometer. The metal-ion is in the d
6 high-spin electronic configuration and it appears to have an octahedral coordination. The thermal variation of the d.c. magnetic susceptibility of Fe2[O3P-(CH2)2-PO3]·2H2O follows the Curie-Weiss law, with a negative Weiss constant of θ =-55K. Below 100K, the susceptibility increases until a peak at TN=24K is observed, and this is associated to the three-dimensional antiferromagnetic order. Below 24K, Fe2[O3P-(CH2)2-PO3]·2H2O behaves as a “weak” ferromagnet and it represents an example of an unusual kind of molecular-based magnetic materials, having a finite zero-field magnetization.