Abstract Single crystals of potassium magnesium phosphate KMg 4(PO 4) 3 have been obtained from flux and characterized by means of single-crystal X-ray diffraction method and IR and Raman spectroscopy. This compound crystallizes in the Pnnm space group with a = 16.361 ( 3 ) , b = 9.562 ( 19 ) , c = 6.171 ( 12 ) Å and Z = 4 . The structure is built up of PO 4 tetrahedra, MgO 6 octahedra and MgO 5 polyhedra connected via P O Mg bridges to form a three-dimensional framework enclosing tunnels along b-axis occupied by K + ions. IR and Raman spectra are consistent with the crystal structure. The internal and external modes are observed in the 350–1210 cm −1 range and below 330 cm −1, respectively.