A numerical model to predict the diffusion and reaction of oxygen in petroleum bitumen films was developed, and compared to experimental data obtained by oxidation of bitumen films at 50 degrees C and atmospheric pressure in the absence of light. Experimental viscosity-depth profiles were obtained by microtoming 3-4 mm thick, oxidized, bitumen films at approximately 100 mu m intervals. Model parameters for the diffusion-oxidation process were obtained by correlating oxygen uptake of bitumen solutions to changes in carbonyl infrared spectral absorption and viscosity. The model developed, predicted the average viscosity in bitumen films after oxidation at 50 degrees C for up to 126 days (approximately equivalent to 2 years in the field) with an accuracy of +/- 20%. (C) 2011 Elsevier Ltd. All rights reserved.