The vanadium–molybdenum mixed oxide bronzes of composition K0.23(V5.35Mo0.65)O15 and K0.32(V5.48Mo0.52)O15 have a monoclinic structure with s.g. C2/m, Z ¼ 2, and unit-cell dimensions a ¼ 15.436(2), b ¼ 3.6527(5), c ¼ 10.150(1)A ˚ , b ¼ 108.604(3)1 and a ¼ 15.452(2), b ¼ 3.6502(5), c ¼ 10.142(1)A ˚ , b ¼ 109.168(3)1, respectively, as determined by single-crystal X-ray diffraction. These compounds show the b-NaxV6O15 tunnel structure, which is isostructural with bannermanite, natural sodium–potassium vanadate. Structure refinements from diffracted intensities collected in the 2–381y range converged to final R ¼ 5.58% and 7.48% for the two crystals, respectively. The V atoms are distributed on three different crystallographic sites. Partial substitution of V with Mo occurs in only one of these positions. Oxygen atoms involved in vanadyl groups point toward the tunnels. The K ions in the tunnels are coordinated by seven oxygen atoms. The alkali metal content in these crystals is much lower than the solubility limit found for the analogous Na containing compound.