The structures found in the 4th stage graphite lamellar compound containing 24.8 % MoC15 have been determined by a combination of single-crystal electron-diffraction and X-ray-diffraction methods. The use of the latter technique showed sets of four adjacent graphite sheets to retain the normal ABAB stacking and interlayer spacing of free graphite. Electron diffraction showed the lateral structure within each graphite sheet to be unchanged and the intercalated molybdenum pentachloride layers to exist in two or more phases. In the most clearly characterized of these phases the dimer molecular structure of the free pentachloride is preserved, the molecules lying in a close-packed array having a precise orientation to the adjacent graphite sheets. The measurable intramolecular atomic distances of the dimer molecule remain unchanged from those in the free chloride but owing to interaction with the graphite the molecular symmetry is lower than that possessed by non-intercalated molecules. The other reactant layer phases are characterized by hexagonal and disordered cation distributions, the dimeric relationship no longer being distinguishable. A substructure consisting of two adjacent pseudo close packed chlorine layers is common to all phases, individual phases being distinguished by the cation distribution within the octahedral sites. Certain orientations of the substructure relative to the graphite are more highly favoured, the reactant at other orientations being characterized by a disordered cation distribution and an increase in the mean chlorine-chlorine distance measured parallel to the layer plane.