In this paper, we develop a first principles atomistic model of single atom substitutions within boron suboxide to predict the effect of potential sintering aides (Al, Mg, Si, and Lu) and processing contaminants (C, N, F, S, P, Li, and Ti) on its cohesive energies and local bonding reconstruction. Our results indicate that metallic dopants strongly destabilize the B6O crystal structure. However, non-metallic dopants, particularly C and N, only slightly weaken the cohesive energy of the crystal. We then performed a more detailed study of multi-atom carbon substitution within B6O. The energetic cost (2.81 eV) associated with doping two carbon atoms was highest when the carbon substituted for two polar boron sites from adjacent icosahedra and lowest when the carbon replaces both an equatorial boron and its neighboring oxygen, forming a Q-Co dimer. When a C-B-C chain replaced an 0-0 chain within the B6O unit cell, the product state was lower in energy than pristine B6O. As more carbon substituted within the icosahedra neighboring the C-B-C chain, the product structure became increasingly lower in energy. This suggested an exothermic reaction pathway for the precipitation of a local boron carbide-like structure from B6O. These results partially explain the driving force behind the formation of B4C that is often encountered during B6O processing. © 2016 by The American Ceramic Society.