The reactivities of six supercritical fractions of asphalt SHRP AAF-1 were investigated in the context of their chemical composition. The supercritical fractions were obtained using pentane as the solvent. Each fraction was aged in a pressure oxygen vessel (POV) under seven conditions of temperature and pressure. As with whole asphalts, there is an initial rapid oxidation that slows to a constant rate. The oxidation rates for the constant rate region were measured at each of the conditions to determine the kinetic parameters of the fractions. Arrhenius plots were then constructed for the fractions to compare their reactivities. Variations in activation energy and reaction order were within experimental err;or. Results show that reactivity increases with fraction number for the supercritical fractions. The heavier fractions are more reactive not only because they contain more polar aromatics in terms of Corbett analysis fractions but also because the polar aromatics of the heavier fractions are larger in molecular size and are more reactive. Additionally, heavier fractions contain more asphaltenes that are larger in molecular size. Because for light fractions the reaction rates, and as previously shown the hardening rates, are very low and remain constant with oxidation, air blowing of some light fractions would be expected to produce an asphalt with a very low hardening rate. Data explicitly show that for petroleum fractions from the same crude source molecular size distribution is a good indicator of reactivity. This is not necessarily true for petroleum fractions from different crude sources, however.