Heavy oil usually contains percentage levels of sulfur. Most of the sulfur in heavy oil is found in bulky thiophenic structures. Thiophenic sulfur is difficult to remove by catalytic hydrodesulfurization, but it can readily be oxidized. The sulfoxides and sulfones produced from sulfur oxidation can be solvent extracted from the heavy oil as a result of their increased polarity. Oxidative desulfurization of heavy oil was studied using Canadian Cold Lake bitumen (5% S, 1100 kg/m(3)), with air as oxidant. At the conditions investigated, namely, autoxidation at 145-175 degrees C followed by water washing, 46-47% of the sulfur in the bitumen could be removed. This is equivalent to >20 kg sulfur per ton oil. Part of the sulfur was removed as SO2 and part as water extracted sulfur-containing compounds. Lower autoxidation temperatures led to better desulfurization. The main challenge was to prevent free radical addition reactions that cause a viscosity increase and bitumen hardening. Autoxidation of undiluted bitumen and bitumen-water mixtures resulted in hardening. Hardening was prevented when bitumen was diluted with naphtha (n-heptane). However, the oxidized sulfur compounds could not be extracted with water from the bitumen-heptane phase, and some material was precipitated as a result of solvent deasphalting. Oxidation selectivity was studied using a model dibenzothiophene and n-heptane mixture. Some precipitation was also observed, and the chemistry was analogous to the precipitation chemistry (gum formation) that undermines storage stability of transportation fuels.