A variety of cokes formed in the reactor and transfer lines of a commercial ethylene dichloride pyrolysis process producing a vinyl chloride monomer were characterized by optical and electron scanning microscopies to determine the mechanism by which they were formed and to find means for suppressing this coke formation. Typical columnar pyrolytic carbon was found on the reactor wall, which was anisotropic carbon of granular appearance. Deposits of carbon found in the transfer lines of product and feed varied in amount and composition, depending on the location of the deposition. The latter carbon appeared to be formed through a mechanism similar to refluxing carbonization of reactive species such as chloroprene and acetylene, which are easily oligomerized, can precipitate on the wall, and finally yield carbon after the repeated dissolution and precipitation. The temperature and kinds of reactive species in the product and feed may define the extent of adhesion, coalescence, and growth of carbon primary granules and may induce their softening during carbonization.