We present a new effective method for synthesizing Sr2MWO6 double perovskite oxides: the co-precipitation route at 1220 K in nitrogen environment. Using conventional X-ray diffraction methods, we have confirmed the room-temperature space group of Sr2Mn2+WO6 to be P2(1)/n: Z = 2; a = 5.6764(1)angstrom; b = 5.6752(1)angstrom; c = 8.0144(1)angstrom; beta = 90.12(1)degrees and V = 258.18(1)angstrom. We show that the compound presents the following temperature induced phase-transition sequence: P2(1)/n -> l4/m -> Fm (3) over barm. After a thermal treatment, 24 h at 870 K in air, the Sr2Mn2+WO6 compound transforms irreversibly to Sr2Mn3+WO6+delta. This transformation has been confirmed by EPR and XANES measurements. Using conventional X-ray diffraction methods, we have shown the room-temperature space group of Sr2Mn3+WO6+delta to be l4/m: Z = 2; a = 5.6353(1) angstrom; c = 8.0149(1) angstrom and V = 254.53(1) angstrom(3). We show that the compound presents the following temperature induced phase-transition sequence: 14/m(-> l4/mmm) -> Fm (3) over barm. The tetragonal-to-tetragonal phase transition is suggested to be present, it is not observed as it is very weak: it is attributed to the presence of the Jahn-Teller active Mn3+ cation. (C) 2009 Elsevier B.V. All rights reserved.