Two new diphosphates, Cd5MIII 2(P2O7)4withM= V, Fe, isotypic with FeII 5FeIII 2(P2O7)4, have been synthesized. They crystallize in the space groupC2221witha∼ 8.8 Å,b∼ 9.9 Å, andc∼ 24.1 Å. The single crystal structure determination of the V phase and the Mössbauer spectroscopy study of the Fe phase, show that the cationic distribution is different from that observed in FeII 5FeIII 2(P2O7)4, i.e., V(III) or Fe(III) is located in two kinds of sites instead of one kind of site. A detailed analysis of this structure shows that its octahedral layers consist of rock salt type ribbons, themselves built up from infinite chains of edge-sharing octahedra, interconnected through trioctahedral units of edge-sharing octahedra. On the basis of this description, the different cationic distribution compared to the pure Fe phase is then explained by the mismatch between infinite octahedral chains and trioctahedral units, due to the larger size of Cd(II) compared to Fe(II).