The oxidation mechanisms of isoprene and butadiene initiated by OH in the presence of NO have been explored under “wall-less” flowing conditions, with products observed a few seconds after reaction by infrared spectroscopy. Since only ∼1% of alkene is reacted, any secondary chemistry is negligible. The use of reaction modulation spectroscopy permits the accurate measurement of a percent change in high alkene concentration and of 10¹³ molecules/cm³ concentrations for products. Measured carbonyl species agree with previous studies, while alkyl nitrate yields are consistent with upper values reported in the literature. NO sensitivity studies performed exclude the possibility of competing chemistry. Isoprene is not observed to form 3-methyl furan, indicating that this is not a prompt oxidation product. However, butadiene does form furan. In an auxiliary experiment, peroxy radicals in the second stage of butadiene oxidation are fully converted to peroxynitrates. Average cross sections for integrated peroxynitrate bands are determined from this experiment.