Single crystal and bulk polycrystalline forms of K2MP2O7 (M=Fe(II), Cu(II)) have been synthesised and their structures determined from single crystal X-ray diffraction data. Both compounds crystallize in the tetragonal system, space group P-421m. Their structures are formed from infinite sheets of linked oxopolyhedra of the stoichiometry [MP2O7]2− with potassium cations situated between the layers. The MO4 tetrahedra share oxygen atoms with [P2O7]4− diphosphate groups and the potassium ions have KO8 square prismatic geometry. In both compounds the M(II) centre has an unusual strongly flattened, tetrahedral coordination to oxygen, as a result of the Jahn–Teller (JT) effect for the high spin d6 Fe(II) and p-orbital mixing or a second order JT effect for d9 Cu(II) centres in four fold coordination. The uncommon transition metal ion environments found in these materials are reflected in their optical absorption spectra and magnetism data.