This study investigated the deposition of coke in a fractionation tower following thermal cracking of heavy feedstocks. A simple microscopic technique was used to determine whether the coke formed in situ in the fractionator or was formed elsewhere (e.g. in the reactor vessel) and was subsequently entrained in the vapour phase. The reflectance of coke types, mode of occurrence, and distributions from the bottom to the top of the fractionator were used to interpret the range of temperatures responsible for coke formation. Both isotropic and anisotropic (mosaic) coke was observed in the samples. The anisotropic textural features indicated that asphaltenes carry-over was a minor problem and that the vast majority of the isotropic coke precursors were the maltenes present in the gas phase that entered the fractionator. The formation of perfectly spherical mesophase was attributed to the gas oil stream itself used to quench the vapours exiting the thermal cracking vessel. Microscopic evidence, along with metals concentration in the coke at various locations of the operation provided useful information as to the nature of the coke precursors. (C) 2003 Elsevier Science Ltd. All rights reserved.