Incipient coke formation was studied during heating of Athabasca pitch (+524 degreesC), heavy gas oils, their mixtures, and both distillation and solvent fractions in isothermal batch reactors at atmospheric pressure. Samples of about 1.5 g were reacted in open glass lined reactor tubes at temperatures from 360 to 420 degreesC over times from 0.3 to 7 h. Toluene insolubles and volatiles were measured in all experiments, with asphaltenes and heptane solubles in selected cases. Kinetic data are presented for toluene insoluble formation; results for pitch appear to follow the Wiehe series model for coke formation. The composition of the toluene insolubles formed is shown to depend on reaction severity. (C) 2004 Elsevier Ltd. All rights reserved.