A complex layered oxide has been synthesised as bulk material starting from the theoretical n = 2 R-P formula Sr3(MgxMo1 − x)2O7 − δ (x ≈ 0.25). This compound exhibits an unexpected crystal chemistry illustrated by the coexistence of several stacking modes — principally the ordered double perovskite Sr2MgMoO6 − δ 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 − δ 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)3Mo2O7]1 − x[Sr2MgMoO6]x with x ≈ 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.