Highly oxygenated organic molecules (HOM) are found to play an important role in the formation and growth of secondary organic aerosol (SOA). SOA is an important type of aerosol with significant impact on air quality and climate. Compared with the oxidation of volatile organic compounds by ozone (O.sub.3) and hydroxyl radical (OH), HOM formation in the oxidation by nitrate radical (NO.sub.3 ), an important oxidant at nighttime and dawn, has received less attention. In this study, HOM formation in the reaction of isoprene with NO.sub.3 was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). A large number of HOM, including monomers (C.sub.5 ), dimers (C.sub.10 ), and trimers (C.sub.15 ), both closed-shell compounds and open-shell peroxy radicals (RO.sub.2 ), were identified and were classified into various series according to their formula. Their formation pathways were proposed based on the peroxy radicals observed and known mechanisms in the literature, which were further constrained by the time profiles of HOM after sequential isoprene addition to differentiate first- and second-generation products. HOM monomers containing one to three N atoms (1-3N-monomers) were formed, starting with NO.sub.3 addition to carbon double bond, forming peroxy radicals, followed by autoxidation. 1N-monomers were formed by both the direct reaction of NO.sub.3 with isoprene and of NO.sub.3 with first-generation products. 2N-monomers (e.g., C.sub.5 H.sub.8 N.sub.2 O.sub.n(n=7-13), C.sub.5 H.sub.10 N.sub.2 O.sub.n(n=8-14)) were likely the termination products of C.sub.5 H.sub.9 N.sub.2 O.sub.n â«, which was formed by the addition of NO.sub.3 to C5-hydroxynitrate (C.sub.5 H.sub.9 NO.sub.4 ), a first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for â¼34 % of all HOM, indicating the important role of second-generation oxidation in HOM formation in the isoprene + NO.sub.3 reaction under our experimental conditions. H shift of alkoxy radicals to form peroxy radicals and subsequent autoxidation ("alkoxy-peroxy" pathway) was found to be an important pathway of HOM formation. HOM dimers were mostly formed by the accretion reaction of various HOM monomer RO.sub.2 and via the termination reactions of dimer RO.sub.2 formed by further reaction of closed-shell dimers with NO.sub.3 and possibly by the reaction of C5-RO.sub.2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO.sub.2 with monomer RO.sub.2 . The concentrations of different HOM showed distinct time profiles during the reaction, which was linked to their formation pathway. HOM concentrations either showed a typical time profile of first-generation products, second-generation products, or a combination of both, indicating multiple formation pathways and/or multiple isomers. Total HOM molar yield was estimated to be 1.2 %-0.7%+1.3%, which corresponded to a SOA yield of â¼3.6 % assuming the molecular weight of C.sub.5 H.sub.9 NO.sub.6 as the lower limit. This yield suggests that HOM may contribute a significant fraction to SOA yield in the reaction of isoprene with NO.sub.3.