Secondary organic aerosol (SOA) formation proceeds by bimolecular gas-phase oxidation reactions generating species that are sufficiently low in volatility to partition into the condensed phase. Advances in instrumentation have revealed that atmospheric SOA is less volatile and more oxidized than can be explained solely by these well-studied gas-phase oxidation pathways, supporting the role of additional chemical processes. These processes-autoxidation, accretion, and organic salt formation-can lead to exceedingly low-volatility species that recently have been identified in laboratory and field studies. Despite these new insights, the identities of the condensing species at the molecular level and the relative importance of the various formation processes remain poorly constrained. The thermodynamics of autoxidation, accretion, and organic salt formation can be described by equilibrium partitioning theory; a framework for which is presented here. This framework will facilitate the inclusion of such processes in model representations of SOA formation.