Viscosity of bitumen solutions in heptol (80:20) and heptane above the onset of asphaltene precipitation decreases continually with aging for up to 30 days, while in toluene, there was no detectable change in the viscosity. The decrease of the viscosity at these conditions was related to the formation of asphaltene aggregates and aggregate dusters, which precipitated out and, as a result, decreased the bitumen (asphaltene) content of the solution. The observed time dependence of the viscosity reduction suggests that the asphaltene aggregation/precipitation is a continuous process with the time scale of weeks. The asphaltene aggregation/precipitation rate depends upon the aromaticity of the diluent, with higher rates observed for heptane compared to heptol (80:20). These results add to the current understanding of the relation between aggregation of asphaltenes and viscosity of diluted bitumen, which is important for improving heavy oil extraction and processing technologies.