Blue single crystals of Cu[µ2-OOC(CH2)PO3H]*2H2O (1) and Cu1.5[µ3-OOC(CH2)PO3]*5H2O (2) have been prepared in aqueous solution. 1: Space group C2/c (no. 15) with a = 1623.3(2), b = 624.0(1), c = 1495.5(2) pm, beta = 122.45(1)°. Cu is coordinated by three oxygen atoms stemming from the hydrogenphosphonoacetate dianion and three water molecules to form a distorted octahedron. The Cu-O bonds range from 190.4(3) to 278.5(3) pm. The connection between the Cu2+ cations and the hydrogenphosphonoacetate dianions leads to a two-dimensional structure with layers parallel to (-101). ̅The layers are linked by hydrogen bonds. 2: Space group P-1 (no. 2) with a = 608.2(1), b = 800.1(1), c = 1083.6(1) pm,alpha = 94.98(1)°, beta = 105.71(1)°, gamma = 109.84(1)°. There are two crystallographically independent Cu2+ cations coordinated in a square pyramidal and an octahedral fashion, respectively. The Cu-O bonds range from 192.9(2) to 237.2(2) pm. The coordination of the phosphonoacetate trianion to Cu(1) results in infinite polyanionic chains parallel to [100] with a composition of {Cu(H2O)[OOC(CH2)PO3]}nn-. Hydrated Cu(2) cations are accommodated between the chains as counter ions. 1 and 2 show structural features of cation exchangers. Magnetic measurements reveal a paramagnetic Curie-Weiss behaviour. Compound 2 shows antiferromagnetic coupling between Cu2+ ions due to a super-superexchange coupling. The UV/Vis spectra of 1 suggests three d-d transition bands at 763 nm (2B1 -> 2E), 878 nm (2B1 -> 2B2), and 1061 nm (2B1 -> 2A1). Thermoanalytical investigations in air show that compound 1 is stable up to 165 °C, whereas the decomposition of 2 begins at 63 °C.