A complex layered oxide has been synthesised as bulk material starting from the theoretical n = 2 R-P formula Sr-3(MgxMo1 - x)(2)O-7 (- delta) (x approximate to 0.25). This compound exhibits an unexpected crystal chemistry illustrated by the coexistence of several stacking modes -principally the ordered double perovskite Sr2MgMoO6 (- delta) and n = 2 R-P type phase - inside the same crystallite. Chemical EDS analyses performed by TEM techniques have lead to an average chemical formula Sr2.68Mg0.64Mo1.67, which differs from the expected n = 2 R-P formula Sr3Mg0.5Mo1.5O7 (-) (delta) in particular through a significant Sr deficiency. From high resolution electron microscopy (HREM) and Z-contrast (SuperSTEM) images, this analytical result is interpreted by the formation of a complex intergrowth [(Sr,Mg)(3)Mo(2)O7](1 -) (x)[Sr2MgMoO6](x) with x approximate to 0.4. The resulting structure can be interpreted as a compositional segregation between Mg2+ and Mo6+ species in two basic structures, namely Sr3Mo2O7 and Sr2MgMoO6. (C) 2010 Elsevier B.V. All rights reserved.