One of the issues for the pipeline transportation of diluted bitumen (dilbit) is the fate and behaviour of the dilbit if it is spilled in freshwater systems; in particular, the evaporation rate and the change in physical properties of the film after evaporation and exposure to water (weathering). In this study, the evaporative weathering of dilbit Cold Lake Winter Blend (CLWB) and light crude oil Alberta Sweet Blend (ASB) films were compared. Evaporation rates were measured for films on glass over time (up to 30 days) at different air-flow rates and at temperatures of 5, 15, and 25 degrees C. The solvent content, density, and viscosity of the films were measured after different weathering times. A second set of tests at 15 degrees C was performed on both dilbit and light-crudeoil films on water. As expected, the mass-transfer rate increased with increasing temperature and decreasing film thickness in all cases. The evaporation of the dilbit was found to be limited by the diffusion rate of the lighter components through the film while that of the light crude oil was limited by convective mass transfer to the air above. The density and viscosity of both the CLWB and ASB films correlated to the amount of evaporated material at all conditions examined, including different film thicknesses, temperatures, and air-flow rates. It appears that the volatile components evaporate in the same order at any conditions; therefore, film composition, density, and viscosity are only functions of the amount evaporated. The evaporation rates, density, and viscosity of CLWB and ASB films weathered over water were identical to those corresponding films weathered on glass, within the error of the measurements; that is, contact with still water had no effect on weathering.