The composition of coke formed on the external surface of H-ZSM-5 during the cracking of hexadecane, at 350 degrees C and 20 atm pressure of hydrogen, was analysed using a combination of pyrolysis mass-spectrometry and transmission electron microscopy. It was shown that the external coke consisted of regions of graphite and areas of polyaromatic hydrocarbons. Pyrolysis at a range of temperatures showed cracking products with up to five aromatic rings. Using standard compounds, it was shown that cracking only occurred at exo-cyclic bonds, and therefore we can say that the regions of amorphous coke consist of large polyaromatic molecules linked by saturated hydrocarbon chains. It would appear that these amorphous regions act as precursors to graphitic coke.