The low-temperature oxidation of oilsands bitumen was investigated to determine how viscosity was affected by oxidation and whether oxidative hardening could be suppressed by solvent dilution. This work was performed to support the development of an oxidative desulfurization process, but it could also find application in processes for bitumen recovery by in situ low-temperature oxidation. The experimental investigation was conducted at 140200 degrees C and near atmospheric pressure with oilsands-derived bitumen, air, and mesitylene as the solvent. Solvent dilution decreased the bitumen viscosity through diluent action, but it did not suppress oxidative hardening of the bitumen. In fact, with the presence of a solvent, the bitumen viscosity increased more than by oxidation of bitumen on its own. This could be explained in terms of easier hydrogen abstraction from bitumen relative to the solvent, which increased the probability of addition reactions. The increase in viscosity with oxidation extent was also investigated, and apparently conflicting reports in the literature were reconciled. At a constant temperature, different periods of near constant viscosity increase with the increase in the oxygen consumption were identified. It was also found that, during the first free radical chain-propagation-dominated oxidation period, the extent of viscosity increase was different at different oxidation temperatures for the same level of O-2 consumption. Oxidative hardening is not just related to oxidation extent, but it is also affected by changes in oxidation selectivity because of the conditions at which the oxidation was performed.