An oxygen-stoichiometric, B-site-ordered perovskite Ca2Fe 1.1Mn0.9O6 (CFMO) with alternate stacking of Fe and Mn layers was obtained through topochemical oxidation of the corresponding brownmillerite phase under high pressure in the presence of KClO4. The structure crystallizes in the P21/m space group with a doubling of the cell along all three crystallographic axes. Mössbauer spectroscopy, susceptibility, and resistivity measurements suggest ferromagnetic interactions between Fe4+(d4) and Mn4+(d3) along  through a double-exchange mechanism, a situation similar to half-doped manganese perovskite oxides. Upon cooling, CFMO exhibits a ferrimagnetic transition below Tc = 90 K, likely accompanied by a charge disproportionation of the iron site, 2Fe4+rarr; Fe3+ + Fe5+. A reasonably good magnetoresistivity of 27 % was observed below Tc. High-pressure oxygen intercalation of a brownmillerite phase Ca2Fe1.1Mn0.9O5 provides a fully oxidized layered double perovskite Ca2Fe1.1Mn 0.9O6 with alternate Fe4+(d4) and Mn4+(d3) layers. Ca2Fe1.1Mn 0.9O6 exhibits charge disproportionation and magneto-resistivity. Electron hopping through double-exchange ferromagnetic interactions between Fe4+ and Mn4+ is proposed. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.