Crystallographic studies of the Ba–Pt–O system have been undertaken using X-ray and electron diffraction techniques. The system is described by means of a Bap(BaxPt2+1−x)Pt4+p−2O3p−3formula which corresponds to a BaO3hexagonal based framework with Pt chains, whereprepresents the oxygen deficiency and the presence of both Pt4+and Pt2+cations in the compounds, andxa possible substitution of Pt2+by Ba2+in trigonal prismatic sites. The structure of a Ba4(Ba0.04Pt2+0.96)Pt4+2O9crystal has been solved by using 5548 X-ray difraction reflections collected on a twinned crystal. Refinements were performed with two distinct models: an “average”P321 space group and an “orthorhombic”C2 space group with cell parametersa=17.460(4) Å,b=10.085(2) Å,c=8.614(3) Å. In this structure, two Pt4+and one Pt2+cations are distributed over four Ba planes and form chains along thecaxis, consisting of two face-sharing Pt4+O6octahedra connected with one Pt2+O6trigonal prism. A lattice misfit occurs between the rigid barium lattice and the PtO3chains, giving rise to a composite structure. Twinning and domain configurations are described and taken into account in the refinement. This twinning is related to the presence of Pt2+cations, whose positions break the threefold axis symmetry. A diffraction anomalous fine structure (DAFS) study was also performed on this twinned single crystal. Anomalous scattering factorsf′ andf″ for platinum in this crystal were refined near the LIIIPt absorption edge. They confirm the weak barium occupancy of the trigonal prismataic site and the Pt4+valence of the octahedral sites. Reflection overlaps, due to twinning, flatten the DAFS sensitivity to Pt atoms in the prismatic sites and did not allow their clear valence determination, but Pt–O bond lengths agree with the presence of Pt2+cations at the center of prismatic faces. Electron diffraction patterns of powders having slightly different composition show a continuous evolution of incommensurate Bragg peaks and a weak correlation between the PtO3chains. They also confirm the composite nature and the one-dimensionality of the Bap(BaxPt2+1−x)Pt4+p−2O3p−3series, which can produce highly anisotropic physical properties.