Isoprene is the largest single VOC emission to the atmosphere. Although it is primarily oxidized photochemi- cally during daylight hours, late-day emissions that remain in the atmosphere at sunset undergo oxidation by NO 3 in regionally polluted areas with large NO x levels. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across the Northeast US, a re- gion with large emissions of both isoprene and NO x . Sub- stantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO 3 and were the most important factor determining the lifetime of this rad- ical. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and served as tracers for the presence of isoprene at sunset, prior to its oxidation by NO 3 . A determination of the mass of isoprene oxidized in darkness showed it to be a large fraction ( > 20%) of emitted isoprene. Organic ni- trates produced from the NO 3 +isoprene reaction, though not directly measured, were estimated to account for 2–9% of to- tal reactive nitrogen. The mass of isoprene oxidized by NO 3 was comparable to and correlated with the organic aerosol loading for flights with relatively low organic aerosol back- ground. The contribution of nocturnal isoprene oxidation to secondary organic aerosol was determined in the range 1– 17%, and isoprene SOA mass derived from NO 3 was calcu- lated to exceed that due to OH by approximately 50%.