The series K x (PO 2) 4(WO 3) 2 m has been investigated by X-ray diffraction and electron microscopy. Five compositions have been isolated as pure phases for x = 2 and m = 4, 6, 7, 8, and 10. These bronzes are built up from WO 3 slabs connected through PO 4 groups forming hexagonal tunnels. High-resolution lattice images allow a correlation between the image contrast and the structure to be established. Three groups of defects are observed in the crystals which deal with (i) the distribution of the phosphate planes with respect to the ReO 3-type slabs, (ii) the distribution of the K + ions in the tunnels, and (iii) the uneven arrangements of the phosphate planes. From this study, it appears that the mechanism of nonstoichiometry is different from that observed in the other phosphate tungsten bronze families according to the existence range and the specific defects which generally involve large parts of the bulk.