Mild pyrolysis (400 degrees C) of bitumen was investigated to establish ways in which coke formation can be suppressed. Bitumen was diluted to various degrees with solvents that had different hydrogen transfer properties, namely, hydrogen donation, hydrogen shuttling, and poor hydrogen transfer properties. Additionally, the concentration of light products generated during bitumen pyrolysis was manipulated by pressure and batch/semibatch operation. Coke formation was suppressed by light material, whether added as a solvent or generated in situ during pyrolysis. As anticipated, hydrogen transfer was important, but coke formation was reduced by 35% at 10% concentration of even a poor hydrogen transfer solvent. Hydrogen availability and the H:C ratio of the reaction mixture were found to be particularly influential in determining whether coke formed. The results showed that light gases produced during pyrolysis were not irreversible reaction products, but continued to participate in the reaction network to moderate the pyrolysis process and suppress coke formation. Applied to industrial operation, evidence was provided to indicate that liquid yield can be increased and coke formation can be suppressed during visbreaking by cofeeding light gases, typically C-4 and lighter hydrocarbons.