Blue single crystals of Cu[µ3-O3P(CH2)2COOH]*2H2O (1) and Cu[(RS)-µ3-O3PCH(C2H5)COOH]*3H2O (2) have been prepared in aqueous Cu2+-solutions (pH = 2.5—3.5) containing 3-phosphonopropionic acid (1) and (RS)-2-phosphonobutyric acid (2), respectively. 1: Space group Pbca (no. 61) with a = 812.5(2), b = 919.00(9), c = 2102.3(2) pm. Cu2+ is five-fold coordinated by three oxygen atoms stemming from [O3P(CH2)2COOH]2— anions and two water molecules. The Cu-O bond lengths range from 194.0(3) to 231.8(4) pm. The connection between the [O3P(CH2)2COOH]2— anions and the Cu2+ cations yields a polymeric structure with layers parallel to (001). The layers are linked by hydrogen bonds. 2: Space group Pbca (no. 61) with a = 1007.17(14), b = 961.2(3), c = 2180.9(4) pm. The copper cations are surrounded by five oxygen atoms in a square pyramidal fashion with Cu-O bonds between 193.6(4) and 236.9(4) pm. The coordination between [O3PCH(C2H5)COOH]2- and Cu2+ results in infinite puckered layers parallel to (001). The layers are not connected by any hydrogen bonds. Each layer contains both R and S isomers of the [O3PCH(C2H5)COOH]2- dianion. Water molecules not bound to Cu2+ are intercalated between the layers. UV/Vis spectra suggest three d-d transition bands at 743, 892, 1016 nm for 1 and four bands at 741, 838, 957 and 1151 nm for 2, respectively. Magnetic measurements suggest a weak antiferromagnetic coupling between Cu2+ due to a super-superexchange interaction. Thermoanalytical investigations in air show that the compounds are stable up to 95 °C (1) and 65 °C (2), respectively.