Abstract The title compound formula, La 3.5Ru 4O 13, has been established by means of crystal structure determination. The orthorhombic unit cell has the following dimensions: a = 11.994, b = 5.609, c = 3.856A˚. Least-squares refinement using single-crystal intensity data corrected for absorption reached R = 0.029 for 777 measured independent reflections. La 3.5Ru 4O 13 is built up from perovskite-type slabs parallel to (100) which are running throughout the crystal; these three octahedra slabs are linked together by either La atoms or RuO 6 octahedra. The latter are also joined together by opposite edges to form one-dimensional infinite strings parallel to the b-axis. The doubling of the c parameter can be caused by the ordering of these RuO 6 chains and La atoms.