A polycrystalline sample of a-FeC20 a . 2D20 has been investigated by neutron diffraction in the temperature range 4.2-300 K. The crystalline structure is monoclinic C2/c with the unit cell constants a = 11.98/~, b = 5.55/~, c = 9.91/~ and /3 = 128.47 ° obtained from data at 300 K. The geometry of two O-D-"-O bonds linking the oxalate infinite chains was established and classification of the hydrogen-bonded water molecule with respect to lone-pairs coordination was proposed. Long-range antiferromagnetic ordering was observed below T N = (16 +--1) K. The magnetic cell is doubled along the a and b axes of the chemical one. The magnetic space group was established to be Sh~ (P2~]). Deduced from the experiment, two spin configurations imply the antiferromagnetic intrachain and interchain coupling within (1 00) planes. The (1 00) planes coupling is provided by pure dipolar interactions. The magnetic moment per Fe 2÷ ion calculated from 4.2 K pattern is (3.7 -0.4)/x B. The 11 ° deviation of magnetic moment direction from the b axis results from a competition between the crystalline field and dipolar anisotropic interactions.