Coke formation under transfer line exchanger conditions, that is, at temperatures from 623 to 873 K and atmospheric pressure, is studied in an electrobalance setup. The coking rate is initially very high (catalytic coking) and drops after a few hours to a constant value. At the studied conditions the observed coking behavior on 15Mo3 alloy pigs can be explained via a catalytic mechanism only, and contributions of the free-radical mechanism and the condensation mechanism are insignificant. Experiments with ethane and naphtha steam cracking effluents and with well-defined reaction mixtures show that the coking rate is independent of the partial pressure of ethene (0-2.7 x 10(4) Pa), ortho-xylene (0-1.0 x 10(4) Pa), heavy aromatic hydrocarbons (0-3.0 x 10(2) Pa), and also 1,3-butadiene (0-2.7 x 10(4) Pa). The rate of coke deposition depends only on the temperature and the ratio of the partial pressures of water to dihydrogen. The activation energy for initial coke formation was estimated to be similar to 90 kJ/mol, a value close to the experimentally determined diffusion energy of carbon in iron.