The structures of the new compound La1.2Sr2.7IrO7.33 and the recently discovered La1.2Sr2.7RuO7.33 have been solved using a combination of X-ray and neutron diffraction. Both compounds crystallize in the trigonal space group R (3) over barm and consist of isolated MO6 (M = Ru, Ir) octahedra, which are arranged in well-defined hexagonal perovskite slabs. These slabs are separated by (Sr2O1+delta) layers containing both O2- and (O-2)(2-) ions. The composition can therefore be written as La1.2Sr2.7MO7-delta(O-2)delta with delta = 0.33. Results of the magnetic susceptibility and XANES measurements show that the transition metal cations are in a pentavalent state. While in La1.2Sr2.7RUO7.33 an antiferromagnetic interaction between the Ru5+ ions is found, La1.2Sr2.7IrO7.33 shows a very small temperature-independent paramagnetism down to 1.8 K due to the strong spin-orbit coupling characteristic for the 5d element iridium.