To identify hydrocarbons that might be sensitive indicators of coke formation in visbreaking, experiments have been conducted on a vacuum residue at 410 degrees C with residence times of up to 60 min in a microreactor. An induction period of 40 min prior to coke formation was observed, consistent with previous laboratory studies, with the changes in bulk aromaticity being small. As an example of trace aliphatic components, the distribution of hopanes proved to be highly sensitive to cracking severity but not to the onset of coke formation. In contrast, the concentrations and distributions of polycyclic aromatic hydrocarbons (PAHs) are sensitive to coke formation. Small ring size PAHs and their substituted equivalents were present in the feed, and their concentrations increased with coke formation. However, for the 6-ring PAH, benzo[ghi]perylene, the concentration was constant at ca. 0.1 ppm of the total aromatics during the coke induction period and then increased sharply thereafter to ca. 0.3 ppm. Therefore, it is proposed that tracing the concentrations of large ring PAHs, such as benzo[ghi]perylene, offers a novel and sensitive approach to assess the onset of coke formation in visbreaking.