Single crystals of the new compound La 1 . 2 Sr 2 . 4 RuO 7 , an oxide related to the hexagonal perovskites, were grown from a BaCl 2 flux. The structure was solved by single crystal X-ray diffraction. La 1 . 2 Sr 2 . 4 RuO 7 crystallizes in the space group R3m with a=5.760(1)A and c=18.273(3)A. It is one of the rare examples of oxides with isolated RuO 6 -octahedra. The structure consists of alternating layers of RuO 6 -octahedra and trigonal (La,Sr)O 6 -prisms. These prisms are capped by one additional oxygen ion, which occupies a distorted position within the (La,Sr)O 6 -layers. La 1 . 2 Sr 2 . 4 RuO 7 is the second member of the general [A 2 O][A n B n - 1 O 3 n ] family of hexagonal perovskites with n=2 and the first ruthenate possessing this structure. XANES investigations, bond valence sum calculations and magnetic measurements show that ruthenium takes the oxidation state +5. Although the ruthenium ions have quite long distances, a medium strong antiferromagnetic interaction between these paramagnetic centers was observed.