The atmospheric aging of soot particles, in which various atmospheric processes alter the particles' chemical and physical properties, is poorly understood and consequently is not well-represented in models. In this work, soot aging via heterogeneous oxidation by OH and ozone is investigated using an aerosol flow reactor coupled to a new high-resolution aerosol mass spectrometric technique that utilizes infrared vaporization and single-photon vacuum ultraviolet ionization. This analytical technique simultaneously measures the elemental and organic carbon components of soot, allowing for the composition of both fractions to be monitored. At oxidant exposures relevant to the particles' atmospheric lifetimes (the equivalent of several days of oxidation), the elemental carbon portion of the soot, which makes up the majority of the particle mass, undergoes no discernible changes in mass or composition. In contrast, the organic carbon (which in the case of methane flame soot is dominated by aliphatic species) is highly reactive, undergoing first the addition of oxygen-containing functional groups and ultimately the loss of organic carbon mass from fragmentation reactions that form volatile products. These changes occur on time scales comparable to those of other nonoxidative aging processes such as condensation, suggesting that further research into the combined effects of heterogeneous and condensational aging is needed to improve our ability to accurately predict the climate and health impacts of soot particles.