Abstract A complex layered oxide has been synthesised as bulk material starting from the theoretical n = 2 R-P formula Sr 3 (Mg x Mo 1 − x ) 2 O 7 − δ ( x ≈ 0.25). This compound exhibits an unexpected crystal chemistry illustrated by the coexistence of several stacking modes — principally the ordered double perovskite Sr 2 MgMoO 6 − 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 Sr 2.68 Mg 0.64 Mo 1.67 , which differs from the expected n = 2 R-P formula Sr 3 Mg 0.5 Mo 1.5 O 7 − 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) 3 Mo 2 O 7 ] 1 − x [Sr 2 MgMoO 6 ] x with x ≈ 0.4. The resulting structure can be interpreted as a compositional segregation between Mg 2+ and Mo 6+ species in two basic structures, namely Sr 3 Mo 2 O 7 and Sr 2 MgMoO 6 .