The conversion of bitumen with ozone should in principle lead to oxidative degradation, as opposed to the increase in viscosity found during the reaction with oxygen because ozonation can lead to C=C scission by ozonolysis. However, when bitumen was reacted with ozonated air at 140-160 degrees C, the viscosity increased compared to conversion with air under the same conditions. To explain these observations, the ozonation of model compounds of various hydrocarbon and heterocyclic compound classes was studied. The selectivity and nature of the oxidation products from ozonation were different from that of autoxidation. Ozonolysis that led to the ring opening of aromatics was observed. At low conversion (<10%), the addition product selectivity of the five -membered cyclic hydrocarbons, indane and indene, was less after reaction with ozonated air than with air. It appears that the observed increase in bitumen viscosity was due to the increased free radical addition product selectivity of heterocyclic compounds, which was higher after reaction with ozonated air than with air. It was also found that at higher conversion (>10%), the increase in oxidized material caused a dramatic increase in addition product selectivity during ozonation. A plausible reaction pathway for addition product formation was proposed on the basis of the Criegee mechanism. The nature of unique addition products from ozonation that were not observed after autoxidation could be explained in terms of the decomposition of the ozonide and recombination of the free radical fragments.