The system CuO-Fe2O3-P2O5 has been investigated by means of the solid state reaction between CuO, Fe2O3 and (NH4)2HPO4 in quartz crucibles at 900 °C. The powder samples were characterized by X-ray diffraction, IR spectroscopy and TG/DTA. Single crystals of a new quaternary phase Cu8Fe2P4O21 were achieved by cooling from the melt of the compound in a sealed, evacuated quartz ampoule. Cu8Fe2O5(PO4)4 crystallizes in the monoclinic space group C2/m (No 12) with a = 15.9733(8) Å, b = 5.9438(3) Å, c = 9.5530(5) Å, β = 113.76(1)°, Z = 2. The three-dimensional framework consists of [FeO6] octahedra, three different [CuO5] polyhedra and [PO4] tetrahedra. Cu8Fe2P4O21 exhibits an incongruently melting point at 945 °C.