The location and composition of the coke formed over H-ZSM-5 zeolites during the cracking of hexadecane at 350 degrees C and at 20 atmospheres was studied. A solvent extraction technique using two different solvents located the coke either on the external surface or within the pores of the zeolite and CC-Ms analysis characterised the soluble coke. Coke was also examined using C-13 MAS NMR and high resolution TEM. Substituted benzenes were identified as the major precursors to coke formation, and the external coke had a graphitic structure. The contact time was shown to affect only the rate of formation but not the composition of the coke.