The coupling of reaction and mass transfer was investigated for the thermal cracking of thin films of an Athabasca vacuum residue. Thin films of the vacuum residue were coated on the interior of a stainless steel tube and then heated to 530 degreesC using induction heating. Vapor products were swept out of the reaction zone with nitrogen, and reactions in the liquid phase were allowed to proceed to completion. Coke yield, total liquid product, and gas make were determined for initial film thicknesses ranging from 10 to 80 mum. Product qualities of the liquid samples were characterized by high-temperature simulated distillation and microcarbon residue concentration. The coke yield decreased as the film thickness was reduced, while the yield of distillate products (< 524 degreesC boiling point) increased. As the film thickness was reduced, the mode of mass transport shifted from bubbling in thick films to diffusion through a stagnant thin film. These results were consistent with a decrease in the trapping of cracked products in coke as the film thickness was reduced, because of more effective transport of products out of the liquid phase. A mathematical model was developed to analyze the results and to estimate the diffusivity of gases in the reacting liquid phase.