Stages 4–6 FeCl3–graphite intercalation compounds (GIC's) have been prepared by an ordinary two-bulb method, and the bi-intercalation processes of H2SO4 into the GIC's have been studied by x-ray diffraction. Since stages 4–6 FeCl3–GIC's have more than three vacant interlayer spaces in the c-axis repeat distance, the bi-intercalation of H2SO4 into the GIC's takes place stepwise. Consequently, various FeCl3–H2SO4–graphite bi-intercalation compounds (GBC's), from the GBC's with one H2SO4 layer to the GBC's saturated with H2SO4, are obtained in the bi-intercalation processes. Their compositions are described as C6.0nFeCl3 · c(6.0/6.5)H2SO4 from the calculation of structure factor for x-ray diffraction, where n is the stage number of the starting FeCl3-GIC's, and c is the number of H2SO4 layers. Stacking sequences of all the FeCl3–H2SO4–GBC's are also determined.
