Diphosphates of the stoichiometry AM3(P2O7)2 (A = alkaline-earth metals; M = Fe, Co, Ni) have been synthesized and structurally characterized by single-crystal X-ray diffraction. Crystal data: CaNi3(P2O7)2, monoclinic, P21/c, a = 7.330(3) Å, b = 7.589(3) Å, c = 9.400(3) Å, β = 111.90(3)∘, V = 485.1(3) Å3, Z = 2, and R = 0.026; CaCo3(P2O7)2, as above except a = 7.394(1) Å, b = 7.6266(9) Å, c = 9.444(2) Å, β= 111.73(2)∘, V= 494.7(1) Å3, and R = 0.035; SrFe3(P2O7)2, as above except a = 7.553(1) Å, b = 7.7477(8) Å, c = 9.5796(8) Å, β = 112.11(1)∘, V = 519.4(1) Å3, and R = 0.029. The three compounds are isostructural, consisting of zigzag infinite chains of MO6 octahedra sharing either trans or skew edges. The infinite chains are connected by P2O7 groups to form a three-dimensional architecture with channels parallel to the b axis. Alkaline-earth metals are located in sites within the channels. The following isostructural compounds have also been prepared: AM3(P2O7)2 (A = Sr, Ba; M = Ni, Co) and BaFe3(P2O7)2. A magnetic susceptibility study on SrFe3(P2O7)2 indicates that the material is paramagnetic with an effective magnetic moment of 5.07 µB per Fe between 300 and 26 K. Below this temperature the magnetic susceptibility increases sharply to a maximum at 6 K and then decreases rapidly. Mossbauer spectra also confirm the presence of Fe(II).