Asphaltenes from Safaniya and Venezuelan crude derived bitumens (S180 and V180, respectively) were recovered using n-heptane. The rate of oxygen uptake for solutions of these asphaltenes in toluene at 50.0 degreesC and constant oxygen concentration (0.00187 M) was measured using a gas chromatographic method. The order of the reaction with respect to asphaltene concentration (in the order of 10(-5) g L-1) based on measurement of initial rates was found to 1.39 +/- 0.2 and 1.13 +/- 0.1 for S180 and V180, respectively. For both bitumens at longer reaction times, the rate becomes first-order in asphaltene concentration. The observed change in the order of reaction is consistent with an inhibited autoxidation mechanism in which initiation in the early stages of the reaction involves formation of an oxygen-asphaltene complex.