The crystal and molecular structure of cobalt(II) oxydiacetate trihydrate, Co(C4H4O5)·3H2O, has been determined from single-crystal, three-dimensional x-ray diffraction counter data. The compound crystallizes as dark red violet platelets in space groupP21/n of the monoclinic system withZ=4 and having cell dimensionsa=7·128(1)A,b=10·384(2)A,c=11·122(1)A, and β=91·51(1)A. The structure was refined toR=0·042 andRw=0·053. The cobalt(II) ion has a distorted octahedral coordination in which the oxydiacetate acts as a tridentate chelate and the three remaingng coordination sites are occupied by two water molecules and an oxygen atom from a carboxylate group of an adjacent oxydiacetate dianion. This latter linkage forms a chain structure. The chains are packed in layers and there is extensive hydrogen bonding between the layers. The magnetic susceptibility of cobalt(II) oxydiacetate trihydrate is dependent on the applied magnetic field. In an applied magnetic field of 1,000 Oe, the maximum susceptibility occurs at 2·51 K, and the inflection point belowTmax lies at 2·40 K. (χmax,Tmax) at 100 Oe and 10,000 Oe are (0·257 emu/mole, 2·63 K) and (0·227, 1·76 K).