The lifetime of alkylperoxy radicals (RO2â¢) formed in the oxidation of volatile organic compounds (VOCs) is a key determinant of reaction mechanisms and products. When RO2⢠radical lifetimes are long, autoxidation reactions can form highly oxidized multifunctional compounds (HOMs) that are efficient at forming secondary organic aerosol (SOA). We measured the formation of HOMs resulting from the O3-initiated autoxidation of limonene emitted inside the University of Colorado Art Museum. Conditions inside the museum favored autoxidation for most of the six-week study, indicating that autoxidation is prevalent indoors in the absence of an indoor combustion source of nitrogen oxide (NO). A box model of the museum was used with measurements of VOCs, O3, and NOx and the air exchange to estimate HOM and SOA yields, and to model the limonene oxidation rate. The HOM molar yield of 11% agrees well with laboratory studies of limonene autoxidation, and the SOA mass yield of 0.47 ± 0.08 indicates that limonene autoxidation efficiently forms SOA indoors.