The kinetics of thermal cracking of films of vacuum residue from Athabasca bitumen in the temperature range of 457-530 degrees C was modelled with liquid-phase mass transfer, reaction-dependent fluid properties, and coke formation by reaction of cracked products in the liquid phase. Previous investigations on the thermal cracking of vacuum residue in thin films showed that at low film thickness (similar to 20 +/- 2 mu m) the coke yield was insensitive to the temperature and heating rate for thin films of bitumen. The coke yield increased with the thickness of the initial film, in the range from 20 to 80 mu m (+/-2 mu m). At the same time, the viscosity of the reacting liquid increased rapidly with time, which would slow down the diffusion of products inside the film. This coupling of transport and reaction would enhance the formation of coke by increasing the rate of recombination reactions. The concept of intrinsic coke is used in a new kinetic model to account for the minimum observed coke formation in thin films. With increasing film thickness, the increasing yield of extrinsic coke is modelled through the change in fluid properties as a function of extent of reaction, which reduces the rate of diffusion in the reacting liquid phase. The model was able to properly account for the insensitivity of coke yield in thin films to reaction temperature and the dependence of coke yield on the thickness of the liquid film. (C) 2007 Elsevier Ltd. All rights reserved.