Turquoise monoclinic single crystals of the novel three-dimensional Cu2[µ8-O3P(CH2)2PO3)]*3.2H2O coordination polymer have been prepared using the silica gel method. Space group C2/m (no. 12) with a = 1483.6(2), b = 668.44(8), c = 436.30(6) pm, beta = 93.28(2)°. The Cu2+ cation is coordinated by four oxygen atoms stemming from the 1,2-ethylenediphosphonate dianions in a square planar manner and two water molecules in the axial positions. The connection between the Cu2+ cations and the [PO3C] units from the 1,2-ethylenediphosphonate dianions leads to layers parallel to (100), which are linked by the ethylene groups to a three-dimensional framework with channel-like voids. The voidsaccommodate water molecules not bound to Cu2+ and extend parallel along [001] with an opening of about 550 x 260 pm. Magnetic measurements reveal an antiferromagnetic behaviour due to a superexchange coupling between Cu2+ ions through an oxygen bridge. The UV-Vis spectrum reveals three d-d transition bands at 694, 774, and 918 nm. The compound can be fully dehydrated by thermal treatment and rehydrated by storage in ambient air.