A new mixed valent chromium-layered oxide, Sr 4.5 Cr 2.5 O 9 , with a structure closely related to the 990201+1201:: structure of the superconducting cuprate TlBa 2ุx La 2ุx Cu 2 O 9 , has been synthesized at normal pressure. This orthorhombic phase (a โซุโฌ 10.7959(2) A > , b โซุโฌ 5.4012(1) A > , and c โซุโฌ 30.0755(6) A > ) consists of the regular intergrowth of [Sr 2 CrO 4 ] 0201 blocks and [(Sr 0.5 Cr 0.5 )Sr 2 CrO 5 ] 1201 blocks. It di4ers from the thallium cuprate by the 1:1 ordering of Sr and Cr species within the intermediate layer of the 991201:: block, so that at this level CrO 4 tetrahedra, containing Cr 6ุ species, are obtained. Moreover, at the boundary between the 991201:: and 990201:: blocks, CrO 6 octahedra alternate with CrO 5 pyramids, both occupied by Cr 3ุ species. The magnetic measurements show an original metamagnetic transition from an antiferromagnetic to a weak ferromagnetic state. These properties are interpreted in terms of strong intralayer and weak interlayer magnetic couplings between Cr 3ุ species through a superexchange mechanism.