The intercalation reaction of a mixture of HNO3 and acetic anhydride to form expandable graphite was investigated. Expandable graphites were prepared from the intercalation compounds (GICs) by rapid heating up to 900 degrees C. The effects of reaction conditions on the quality of the expandable graphites were studied and the optimum conditions for the procedure were as follows: weight ratio of natural flake graphite:acetic anhydride:nitric acid:potassium permanganate, 1.0:0.8:0.5:0.07; reaction temperature, 30 degrees C; and time, 40 min. X-ray diffraction patterns showed the intercalation product was a stage 4 compound. Mass spectra showed that nitric and acetic acids were intercalated into graphite layers. Expandable graphite with 280 ml/g expansion volume and 1.27% nitrogen was obtained.